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Natural Selection -- its power compared with man's selection -- its poweron characters of trifling importance -- its power at all ages and on bothsexes -- Sexual Selection -- On the generality of intercrosses betweenindividuals of the same species -- Circumstances favourable andunfavourable to the results of Natural Selection, namely, intercrossing,isolation, number of individuals -- Slow action -- Extinction caused byNatural Selection -- Divergence of Character, related to the diversity ofinhabitants of any small area and to naturalisation -- Action of NaturalSelection, through Divergence of Character and Extinction, on thedescendants from a common parent -- Explains the Grouping of all organicbeings -- Advance in organisation -- Low forms preserved -- Convergence ofcharacter -- Indefinite multiplication of species -- Summary.

How will the struggle for existence, briefly discussed in the last chapter,act in regard to variation? Can the principle of selection, which we haveseen is so potent in the hands of man, apply under nature? I think weshall see that it can act most efficiently. Let the endless number ofslight variations and individual differences occurring in our domesticproductions, and, in a lesser degree, in those under nature, be borne inmind; as well as the strength of the hereditary tendency. Underdomestication, it may truly be said that the whole organisation becomes insome degree plastic. But the variability, which we almost universally meetwith in our domestic productions is not directly produced, as Hooker andAsa Gray have well remarked, by man; he can neither originate varieties norprevent their occurrence; he can only preserve and accumulate such as dooccur. Unintentionally he exposes organic beings to new and changingconditions of life, and variability ensues; but similar changes ofconditions might and do occur under nature. Let it also be borne in mindhow infinitely complex and close-fitting are the mutual relations of allorganic beings to each other and to their physical conditions of life; andconsequently what infinitely varied diversities of structure might be ofuse to each being under changing conditions of life. Can it then bethought improbable, seeing that variations useful to man have undoubtedlyoccurred, that other variations useful in some way to each being in thegreat and complex battle of life, should occur in the course of manysuccessive generations? If such do occur, can we doubt (remembering thatmany more individuals are born than can possibly survive) that individualshaving any advantage, however slight, over others, would have the bestchance of surviving and procreating their kind? On the other hand, we mayfeel sure that any variation in the least degree injurious would be rigidlydestroyed. This preservation of favourable individual differences andvariations, and the destruction of those which are injurious, I have calledNatural Selection, or the Survival of the Fittest. Variations neitheruseful nor injurious would not be affected by natural selection, and wouldbe left either a fluctuating element, as perhaps we see in certainpolymorphic species, or would ultimately become fixed, owing to the natureof the organism and the nature of the conditions.

Several writers have misapprehended or objected to the term NaturalSelection. Some have even imagined that natural selection inducesvariability, whereas it implies only the preservation of such variations asarise and are beneficial to the being under its conditions of life. No oneobjects to agriculturists speaking of the potent effects of man'sselection; and in this case the individual differences given by nature,which man for some object selects, must of necessity first occur. Othershave objected that the term selection implies conscious choice in theanimals which become modified; and it has even been urged that, as plantshave no volition, natural selection is not applicable to them! In theliteral sense of the word, no doubt, natural selection is a false term; butwho ever objected to chemists speaking of the elective affinities of thevarious elements?--and yet an acid cannot strictly be said to elect thebase with which it in preference combines. It has been said that I speakof natural selection as an active power or Deity; but who objects to anauthor speaking of the attraction of gravity as ruling the movements of theplanets? Every one knows what is meant and is implied by such metaphoricalexpressions; and they are almost necessary for brevity. So again it isdifficult to avoid personifying the word Nature; but I mean by nature, onlythe aggregate action and product of many natural laws, and by laws thesequence of events as ascertained by us. With a little familiarity suchsuperficial objections will be forgotten.

We shall best understand the probable course of natural selection by takingthe case of a country undergoing some slight physical change, for instance,of climate. The proportional numbers of its inhabitants will almostimmediately undergo a change, and some species will probably becomeextinct. We may conclude, from what we have seen of the intimate andcomplex manner in which the inhabitants of each country are bound together,that any change in the numerical proportions of the inhabitants,independently of the change of climate itself, would seriously affect theothers. If the country were open on its borders, new forms would certainlyimmigrate, and this would likewise seriously disturb the relations of someof the former inhabitants. Let it be remembered how powerful the influenceof a single introduced tree or mammal has been shown to be. But in thecase of an island, or of a country partly surrounded by barriers, intowhich new and better adapted forms could not freely enter, we should thenhave places in the economy of nature which would assuredly be better filledup if some of the original inhabitants were in some manner modified; for,had the area been open to immigration, these same places would have beenseized on by intruders. In such cases, slight modifications, which in anyway favoured the individuals of any species, by better adapting them totheir altered conditions, would tend to be preserved; and natural selectionwould have free scope for the work of improvement.

We have good reason to believe, as shown in the first chapter, that changesin the conditions of life give a tendency to increased variability; and inthe foregoing cases the conditions the changed, and this would manifestlybe favourable to natural selection, by affording a better chance of theoccurrence of profitable variations. Unless such occur, natural selectioncan do nothing. Under the term of "variations," it must never be forgottenthat mere individual differences are included. As man can produce a greatresult with his domestic animals and plants by adding up in any givendirection individual differences, so could natural selection, but far moreeasily from having incomparably longer time for action. Nor do I believethat any great physical change, as of climate, or any unusual degree ofisolation, to check immigration, is necessary in order that new andunoccupied places should be left for natural selection to fill up byimproving some of the varying inhabitants. For as all the inhabitants ofeach country are struggling together with nicely balanced forces, extremelyslight modifications in the structure or habits of one species would oftengive it an advantage over others; and still further modifications of thesame kind would often still further increase the advantage, as long as thespecies continued under the same conditions of life and profited by similarmeans of subsistence and defence. No country can be named in which all thenative inhabitants are now so perfectly adapted to each other and to thephysical conditions under which they live, that none of them could be stillbetter adapted or improved; for in all countries, the natives have been sofar conquered by naturalised productions that they have allowed someforeigners to take firm possession of the land. And as foreigners havethus in every country beaten some of the natives, we may safely concludethat the natives might have been modified with advantage, so as to havebetter resisted the intruders.

As man can produce, and certainly has produced, a great result by hismethodical and unconscious means of selection, what may not naturalselection effect? Man can act only on external and visible characters: Nature, if I may be allowed to personify the natural preservation orsurvival of the fittest, cares nothing for appearances, except in so far asthey are useful to any being. She can act on every internal organ, onevery shade of constitutional difference, on the whole machinery of life. Man selects only for his own good; Nature only for that of the being whichshe tends. Every selected character is fully exercised by her, as isimplied by the fact of their selection. Man keeps the natives of manyclimates in the same country. He seldom exercises each selected characterin some peculiar and fitting manner; he feeds a long and a short-beakedpigeon on the same food; he does not exercise a long-backed or long-leggedquadruped in any peculiar manner; he exposes sheep with long and short woolto the same climate; does not allow the most vigorous males to struggle forthe females; he does not rigidly destroy all inferior animals, but protectsduring each varying season, as far as lies in his power, all hisproductions. He often begins his selection by some half-monstrous form, orat least by some modification prominent enough to catch the eye or to beplainly useful to him. Under nature, the slightest differences ofstructure or constitution may well turn the nicely-balanced scale in thestruggle for life, and so be preserved. How fleeting are the wishes andefforts of man! How short his time, and consequently how poor will be hisresults, compared with those accumulated by Nature during whole geologicalperiods! Can we wonder, then, that Nature's productions should be far"truer" in character than man's productions; that they should be infinitelybetter adapted to the most complex conditions of life, and should plainlybear the stamp of far higher workmanship?

It may metaphorically be said that natural selection is daily and hourlyscrutinising, throughout the world, the slightest variations; rejectingthose that are bad, preserving and adding up all that are good; silentlyand insensibly working, WHENEVER AND WHEREVER OPPORTUNITY OFFERS, at theimprovement of each organic being in relation to its organic and inorganicconditions of life. We see nothing of these slow changes in progress,until the hand of time has marked the long lapse of ages, and then soimperfect is our view into long-past geological ages that we see only thatthe forms of life are now different from what they formerly were.

In order that any great amount of modification should be effected in aspecies, a variety, when once formed must again, perhaps after a longinterval of time, vary or present individual differences of the samefavourable nature as before; and these must again be preserved, and soonward, step by step. Seeing that individual differences of the same kindperpetually recur, this can hardly be considered as an unwarrantableassumption. But whether it is true, we can judge only by seeing how farthe hypothesis accords with and explains the general phenomena of nature.On the other hand, the ordinary belief that the amount of possiblevariation is a strictly limited quantity, is likewise a simple assumption.

Although natural selection can act only through and for the good of eachbeing, yet characters and structures, which we are apt to consider as ofvery trifling importance, may thus be acted on. When we see leaf-eatinginsects green, and bark-feeders mottled-grey; the alpine ptarmigan white inwinter, the red-grouse the colour of heather, we must believe that thesetints are of service to these birds and insects in preserving them fromdanger. Grouse, if not destroyed at some period of their lives, wouldincrease in countless numbers; they are known to suffer largely from birdsof prey; and hawks are guided by eyesight to their prey,--so much so thaton parts of the continent persons are warned not to keep white pigeons, asbeing the most liable to destruction. Hence natural selection might beeffective in giving the proper colour to each kind of grouse, and inkeeping that colour, when once acquired, true and constant. Nor ought weto think that the occasional destruction of an animal of any particularcolour would produce little effect; we should remember how essential it isin a flock of white sheep to destroy a lamb with the faintest trace ofblack. We have seen how the colour of hogs, which feed on the "paint-root"in Virginia, determines whether they shall live or die. In plants, thedown on the fruit and the colour of the flesh are considered by botanistsas characters of the most trifling importance; yet we hear from anexcellent horticulturist, Downing, that in the United States smooth-skinnedfruits suffer far more from a beetle, a Curculio, than those with down;that purple plums suffer far more from a certain disease than yellow plums;whereas another disease attacks yellow-fleshed peaches far more than thosewith other coloured flesh. If, with all the aids of art, these slightdifferences make a great difference in cultivating the several varieties,assuredly, in a state of nature, where the trees would have to strugglewith other trees and with a host of enemies, such differences wouldeffectually settle which variety, whether a smooth or downy, a yellow or apurple-fleshed fruit, should succeed.

In looking at many small points of difference between species, which, asfar as our ignorance permits us to judge, seem quite unimportant, we mustnot forget that climate, food, etc., have no doubt produced some directeffect. It is also necessary to bear in mind that, owing to the law ofcorrelation, when one part varies and the variations are accumulatedthrough natural selection, other modifications, often of the mostunexpected nature, will ensue.

As we see that those variations which, under domestication, appear at anyparticular period of life, tend to reappear in the offspring at the sameperiod; for instance, in the shape, size and flavour of the seeds of themany varieties of our culinary and agricultural plants; in the caterpillarand cocoon stages of the varieties of the silkworm; in the eggs of poultry,and in the colour of the down of their chickens; in the horns of our sheepand cattle when nearly adult; so in a state of nature natural selectionwill be enabled to act on and modify organic beings at any age, by theaccumulation of variations profitable at that age, and by their inheritanceat a corresponding age. If it profit a plant to have its seeds more andmore widely disseminated by the wind, I can see no greater difficulty inthis being effected through natural selection, than in the cotton-planterincreasing and improving by selection the down in the pods on hiscotton-trees. Natural selection may modify and adapt the larva of aninsect to a score of contingencies, wholly different from those whichconcern the mature insect; and these modifications may affect, throughcorrelation, the structure of the adult. So, conversely, modifications inthe adult may affect the structure of the larva; but in all cases naturalselection will ensure that they shall not be injurious: for if they wereso, the species would become extinct.

Natural selection will modify the structure of the young in relation to theparent and of the parent in relation to the young. In social animals itwill adapt the structure of each individual for the benefit of the wholecommunity; if the community profits by the selected change. What naturalselection cannot do, is to modify the structure of one species, withoutgiving it any advantage, for the good of another species; and thoughstatements to this effect may be found in works of natural history, Icannot find one case which will bear investigation. A structure used onlyonce in an animal's life, if of high importance to it, might be modified toany extent by natural selection; for instance, the great jaws possessed bycertain insects, used exclusively for opening the cocoon--or the hard tipto the beak of unhatched birds, used for breaking the eggs. It has beenasserted, that of the best short-beaked tumbler-pigeons a greater numberperish in the egg than are able to get out of it; so that fanciers assistin the act of hatching. Now, if nature had to make the beak of afull-grown pigeon very short for the bird's own advantage, the process ofmodification would be very slow, and there would be simultaneously the mostrigorous selection of all the young birds within the egg, which had themost powerful and hardest beaks, for all with weak beaks would inevitablyperish: or, more delicate and more easily broken shells might be selected,the thickness of the shell being known to vary like every other structure.

It may be well here to remark that with all beings there must be muchfortuitous destruction, which can have little or no influence on the courseof natural selection. For instance, a vast number of eggs or seeds areannually devoured, and these could be modified through natural selectiononly if they varied in some manner which protected them from their enemies.Yet many of these eggs or seeds would perhaps, if not destroyed, haveyielded individuals better adapted to their conditions of life than any ofthose which happened to survive. So again a vast number of mature animalsand plants, whether or not they be the best adapted to their conditions,must be annually destroyed by accidental causes, which would not be in theleast degree mitigated by certain changes of structure or constitutionwhich would in other ways be beneficial to the species. But let thedestruction of the adults be ever so heavy, if the number which can existin any district be not wholly kept down by such causes--or again let thedestruction of eggs or seeds be so great that only a hundredth or athousandth part are developed--yet of those which do survive, the bestadapted individuals, supposing that there is any variability in afavourable direction, will tend to propagate their kind in larger numbersthan the less well adapted. If the numbers be wholly kept down by thecauses just indicated, as will often have been the case, natural selectionwill be powerless in certain beneficial directions; but this is no validobjection to its efficiency at other times and in other ways; for we arefar from having any reason to suppose that many species ever undergomodification and improvement at the same time in the same area.

SEXUAL SELECTION.

Inasmuch as peculiarities often appear under domestication in one sex andbecome hereditarily attached to that sex, so no doubt it will be undernature. Thus it is rendered possible for the two sexes to be modifiedthrough natural selection in relation to different habits of life, as issometimes the case; or for one sex to be modified in relation to the othersex, as commonly occurs. This leads me to say a few words on what I havecalled sexual selection. This form of selection depends, not on a strugglefor existence in relation to other organic beings or to externalconditions, but on a struggle between the individuals of one sex, generallythe males, for the possession of the other sex. The result is not death tothe unsuccessful competitor, but few or no offspring. Sexual selection is,therefore, less rigorous than natural selection. Generally, the mostvigorous males, those which are best fitted for their places in nature,will leave most progeny. But in many cases victory depends not so much ongeneral vigour, but on having special weapons, confined to the male sex. Ahornless stag or spurless cock would have a poor chance of leaving numerousoffspring. Sexual selection, by always allowing the victor to breed, mightsurely give indomitable courage, length of spur, and strength to the wingto strike in the spurred leg, in nearly the same manner as does the brutalcockfighter by the careful selection of his best cocks. How low in thescale of nature the law of battle descends I know not; male alligators havebeen described as fighting, bellowing, and whirling round, like Indians ina war-dance, for the possession of the females; male salmons have beenobserved fighting all day long; male stag-beetles sometimes bear woundsfrom the huge mandibles of other males; the males of certain hymenopterousinsects have been frequently seen by that inimitable observer M. Fabre,fighting for a particular female who sits by, an apparently unconcernedbeholder of the struggle, and then retires with the conqueror. The war is,perhaps, severest between the males of polygamous animals, and these seemoftenest provided with special weapons. The males of carnivorous animalsare already well armed; though to them and to others, special means ofdefence may be given through means of sexual selection, as the mane of thelion, and the hooked jaw to the male salmon; for the shield may be asimportant for victory as the sword or spear.

Among birds, the contest is often of a more peaceful character. All thosewho have attended to the subject, believe that there is the severestrivalry between the males of many species to attract, by singing, thefemales. The rock-thrush of Guiana, birds of paradise, and some others,congregate, and successive males display with the most elaborate care, andshow off in the best manner, their gorgeous plumage; they likewise performstrange antics before the females, which, standing by as spectators, atlast choose the most attractive partner. Those who have closely attendedto birds in confinement well know that they often take individualpreferences and dislikes: thus Sir R. Heron has described how a piedpeacock was eminently attractive to all his hen birds. I cannot here enteron the necessary details; but if man can in a short time give beauty and anelegant carriage to his bantams, according to his standard of beauty, I cansee no good reason to doubt that female birds, by selecting, duringthousands of generations, the most melodious or beautiful males, accordingto their standard of beauty, might produce a marked effect. Somewell-known laws, with respect to the plumage of male and female birds, incomparison with the plumage of the young, can partly be explained throughthe action of sexual selection on variations occurring at different ages,and transmitted to the males alone or to both sexes at corresponding ages;but I have not space here to enter on this subject.

Thus it is, as I believe, that when the males and females of any animalhave the same general habits of life, but differ in structure, colour, orornament, such differences have been mainly caused by sexual selection: that is, by individual males having had, in successive generations, someslight advantage over other males, in their weapons, means of defence, orcharms; which they have transmitted to their male offspring alone. Yet, Iwould not wish to attribute all sexual differences to this agency: for wesee in our domestic animals peculiarities arising and becoming attached tothe male sex, which apparently have not been augmented through selection byman. The tuft of hair on the breast of the wild turkey-cock cannot be ofany use, and it is doubtful whether it can be ornamental in the eyes of thefemale bird; indeed, had the tuft appeared under domestication it wouldhave been called a monstrosity.

ILLUSTRATIONS OF THE ACTION OF NATURAL SELECTION, OR THE SURVIVAL OF THEFITTEST.

In order to make it clear how, as I believe, natural selection acts, I mustbeg permission to give one or two imaginary illustrations. Let us take thecase of a wolf, which preys on various animals, securing some by craft,some by strength, and some by fleetness; and let us suppose that thefleetest prey, a deer for instance, had from any change in the countryincreased in numbers, or that other prey had decreased in numbers, duringthat season of the year when the wolf was hardest pressed for food. Undersuch circumstances the swiftest and slimmest wolves have the best chance ofsurviving, and so be preserved or selected, provided always that theyretained strength to master their prey at this or some other period of theyear, when they were compelled to prey on other animals. I can see no morereason to doubt that this would be the result, than that man should be ableto improve the fleetness of his greyhounds by careful and methodicalselection, or by that kind of unconscious selection which follows from eachman trying to keep the best dogs without any thought of modifying thebreed. I may add that, according to Mr. Pierce, there are two varieties ofthe wolf inhabiting the Catskill Mountains, in the United States, one witha light greyhound-like form, which pursues deer, and the other more bulky,with shorter legs, which more frequently attacks the shepherd's flocks.

Even without any change in the proportional numbers of the animals on whichour wolf preyed, a cub might be born with an innate tendency to pursuecertain kinds of prey. Nor can this be thought very improbable; for weoften observe great differences in the natural tendencies of our domesticanimals; one cat, for instance, taking to catch rats, another mice; onecat, according to Mr. St. John, bringing home winged game, another hares orrabbits, and another hunting on marshy ground and almost nightly catchingwoodcocks or snipes. The tendency to catch rats rather than mice is knownto be inherited. Now, if any slight innate change of habit or of structurebenefited an individual wolf, it would have the best chance of survivingand of leaving offspring. Some of its young would probably inherit thesame habits or structure, and by the repetition of this process, a newvariety might be formed which would either supplant or coexist with theparent-form of wolf. Or, again, the wolves inhabiting a mountainousdistrict, and those frequenting the lowlands, would naturally be forced tohunt different prey; and from the continued preservation of the individualsbest fitted for the two sites, two varieties might slowly be formed. Thesevarieties would cross and blend where they met; but to this subject ofintercrossing we shall soon have to return. I may add, that, according toMr. Pierce, there are two varieties of the wolf inhabiting the CatskillMountains in the United States, one with a light greyhound-like form, whichpursues deer, and the other more bulky, with shorter legs, which morefrequently attacks the shepherd's flocks.

It should be observed that in the above illustration, I speak of theslimmest individual wolves, and not of any single strongly marked variationhaving been preserved. In former editions of this work I sometimes spokeas if this latter alternative had frequently occurred. I saw the greatimportance of individual differences, and this led me fully to discuss theresults of unconscious selection by man, which depends on the preservationof all the more or less valuable individuals, and on the destruction of theworst. I saw, also, that the preservation in a state of nature of anyoccasional deviation of structure, such as a monstrosity, would be a rareevent; and that, if at first preserved, it would generally be lost bysubsequent intercrossing with ordinary individuals. Nevertheless, untilreading an able and valuable article in the "North British Review" (1867),I did not appreciate how rarely single variations, whether slight orstrongly marked, could be perpetuated. The author takes the case of a pairof animals, producing during their lifetime two hundred offspring, ofwhich, from various causes of destruction, only two on an average surviveto pro-create their kind. This is rather an extreme estimate for most ofthe higher animals, but by no means so for many of the lower organisms. Hethen shows that if a single individual were born, which varied in somemanner, giving it twice as good a chance of life as that of the otherindividuals, yet the chances would be strongly against its survival. Supposing it to survive and to breed, and that half its young inherited thefavourable variation; still, as the Reviewer goes onto show, the youngwould have only a slightly better chance of surviving and breeding; andthis chance would go on decreasing in the succeeding generations. Thejustice of these remarks cannot, I think, be disputed. If, for instance, abird of some kind could procure its food more easily by having its beakcurved, and if one were born with its beak strongly curved, and whichconsequently flourished, nevertheless there would be a very poor chance ofthis one individual perpetuating its kind to the exclusion of the commonform; but there can hardly be a doubt, judging by what we see taking placeunder domestication, that this result would follow from the preservationduring many generations of a large number of individuals with more or lessstrongly curved beaks, and from the destruction of a still larger numberwith the straightest beaks.

It should not, however, be overlooked that certain rather strongly markedvariations, which no one would rank as mere individual differences,frequently recur owing to a similar organisation being similarly acted on--of which fact numerous instances could be given with our domesticproductions. In such cases, if the varying individual did not actuallytransmit to its offspring its newly-acquired character, it wouldundoubtedly transmit to them, as long as the existing conditions remainedthe same, a still stronger tendency to vary in the same manner. There canalso be little doubt that the tendency to vary in the same manner has oftenbeen so strong that all the individuals of the same species have beensimilarly modified without the aid of any form of selection. Or only athird, fifth, or tenth part of the individuals may have been thus affected,of which fact several instances could be given. Thus Graba estimates thatabout one-fifth of the guillemots in the Faroe Islands consist of a varietyso well marked, that it was formerly ranked as a distinct species under thename of Uria lacrymans. In cases of this kind, if the variation were of abeneficial nature, the original form would soon be supplanted by themodified form, through the survival of the fittest.

To the effects of intercrossing in eliminating variations of all kinds, Ishall have to recur; but it may be here remarked that most animals andplants keep to their proper homes, and do not needlessly wander about; wesee this even with migratory birds, which almost always return to the samespot. Consequently each newly-formed variety would generally be at firstlocal, as seems to be the common rule with varieties in a state of nature;so that similarly modified individuals would soon exist in a small bodytogether, and would often breed together. If the new variety weresuccessful in its battle for life, it would slowly spread from a centraldistrict, competing with and conquering the unchanged individuals on themargins of an ever-increasing circle.

It may be worth while to give another and more complex illustration of theaction of natural selection. Certain plants excrete sweet juice,apparently for the sake of eliminating something injurious from the sap: this is effected, for instance, by glands at the base of the stipules insome Leguminosae, and at the backs of the leaves of the common laurel. This juice, though small in quantity, is greedily sought by insects; buttheir visits do not in any way benefit the plant. Now, let us suppose thatthe juice or nectar was excreted from the inside of the flowers of acertain number of plants of any species. Insects in seeking the nectarwould get dusted with pollen, and would often transport it from one flowerto another. The flowers of two distinct individuals of the same specieswould thus get crossed; and the act of crossing, as can be fully proved,gives rise to vigorous seedlings, which consequently would have the bestchance of flourishing and surviving. The plants which produced flowerswith the largest glands or nectaries, excreting most nectar, would oftenestbe visited by insects, and would oftenest be crossed; and so in thelong-run would gain the upper hand and form a local variety. The flowers,also, which had their stamens and pistils placed, in relation to the sizeand habits of the particular insect which visited them, so as to favour inany degree the transportal of the pollen, would likewise be favoured. Wemight have taken the case of insects visiting flowers for the sake ofcollecting pollen instead of nectar; and as pollen is formed for the solepurpose of fertilisation, its destruction appears to be a simple loss tothe plant; yet if a little pollen were carried, at first occasionally andthen habitually, by the pollen-devouring insects from flower to flower, anda cross thus effected, although nine-tenths of the pollen were destroyed itmight still be a great gain to the plant to be thus robbed; and theindividuals which produced more and more pollen, and had larger anthers,would be selected.

When our plant, by the above process long continued, had been renderedhighly attractive to insects, they would, unintentionally on their part,regularly carry pollen from flower to flower; and that they do thiseffectually I could easily show by many striking facts. I will give onlyone, as likewise illustrating one step in the separation of the sexes ofplants. Some holly-trees bear only male flowers, which have four stamensproducing a rather small quantity of pollen, and a rudimentary pistil;other holly-trees bear only female flowers; these have a full-sized pistil,and four stamens with shrivelled anthers, in which not a grain of pollencan be detected. Having found a female tree exactly sixty yards from amale tree, I put the stigmas of twenty flowers, taken from differentbranches, under the microscope, and on all, without exception, there were afew pollen-grains, and on some a profusion. As the wind had set forseveral days from the female to the male tree, the pollen could not thushave been carried. The weather had been cold and boisterous and thereforenot favourable to bees, nevertheless every female flower which I examinedhad been effectually fertilised by the bees, which had flown from tree totree in search of nectar. But to return to our imaginary case; as soon asthe plant had been rendered so highly attractive to insects that pollen wasregularly carried from flower to flower, another process might commence. No naturalist doubts the advantage of what has been called the"physiological division of labour;" hence we may believe that it would beadvantageous to a plant to produce stamens alone in one flower or on onewhole plant, and pistils alone in another flower or on another plant. Inplants under culture and placed under new conditions of life, sometimes themale organs and sometimes the female organs become more or less impotent;now if we suppose this to occur in ever so slight a degree under nature,then, as pollen is already carried regularly from flower to flower, and asa more complete separation of the sexes of our plant would be advantageouson the principle of the division of labour, individuals with this tendencymore and more increased, would be continually favoured or selected, untilat last a complete separation of the sexes might be effected. It wouldtake up too much space to show the various steps, through dimorphism andother means, by which the separation of the sexes in plants of variouskinds is apparently now in progress; but I may add that some of the speciesof holly in North America are, according to Asa Gray, in an exactlyintermediate condition, or, as he expresses it, are more or lessdioeciously polygamous.

Let us now turn to the nectar-feeding insects; we may suppose the plant ofwhich we have been slowly increasing the nectar by continued selection, tobe a common plant; and that certain insects depended in main part on itsnectar for food. I could give many facts showing how anxious bees are tosave time: for instance, their habit of cutting holes and sucking thenectar at the bases of certain flowers, which with a very little moretrouble they can enter by the mouth. Bearing such facts in mind, it may bebelieved that under certain circumstances individual differences in thecurvature or length of the proboscis, etc., too slight to be appreciated byus, might profit a bee or other insect, so that certain individuals wouldbe able to obtain their food more quickly than others; and thus thecommunities to which they belonged would flourish and throw off many swarmsinheriting the same peculiarities. The tubes of the corolla of the commonred or incarnate clovers (Trifolium pratense and incarnatum) do not on ahasty glance appear to differ in length; yet the hive-bee can easily suckthe nectar out of the incarnate clover, but not out of the common redclover, which is visited by humble-bees alone; so that whole fields of thered clover offer in vain an abundant supply of precious nectar to thehive-bee. That this nectar is much liked by the hive-bee is certain; for Ihave repeatedly seen, but only in the autumn, many hive-bees sucking theflowers through holes bitten in the base of the tube by humble bees. Thedifference in the length of the corolla in the two kinds of clover, whichdetermines the visits of the hive-bee, must be very trifling; for I havebeen assured that when red clover has been mown, the flowers of the secondcrop are somewhat smaller, and that these are visited by many hive-bees. Ido not know whether this statement is accurate; nor whether anotherpublished statement can be trusted, namely, that the Ligurian bee, which isgenerally considered a mere variety of the common hive-bee, and whichfreely crosses with it, is able to reach and suck the nectar of the redclover. Thus, in a country where this kind of clover abounded, it might bea great advantage to the hive-bee to have a slightly longer or differentlyconstructed proboscis. On the other hand, as the fertility of this cloverabsolutely depends on bees visiting the flowers, if humble-bees were tobecome rare in any country, it might be a great advantage to the plant tohave a shorter or more deeply divided corolla, so that the hive-bees shouldbe enabled to suck its flowers. Thus I can understand how a flower and abee might slowly become, either simultaneously or one after the other,modified and adapted to each other in the most perfect manner, by thecontinued preservation of all the individuals which presented slightdeviations of structure mutually favourable to each other.

I am well aware that this doctrine of natural selection, exemplified in theabove imaginary instances, is open to the same objections which were firsturged against Sir Charles Lyell's noble views on "the modern changes of theearth, as illustrative of geology;" but we now seldom hear the agencieswhich we see still at work, spoken of as trifling and insignificant, whenused in explaining the excavation of the deepest valleys or the formationof long lines of inland cliffs. Natural selection acts only by thepreservation and accumulation of small inherited modifications, eachprofitable to the preserved being; and as modern geology has almostbanished such views as the excavation of a great valley by a singlediluvial wave, so will natural selection banish the belief of the continuedcreation of new organic beings, or of any great and sudden modification intheir structure.

ON THE INTERCROSSING OF INDIVIDUALS.

I must here introduce a short digression. In the case of animals andplants with separated sexes, it is of course obvious that two individualsmust always (with the exception of the curious and not well understoodcases of parthenogenesis) unite for each birth; but in the case ofhermaphrodites this is far from obvious. Nevertheless there is reason tobelieve that with all hermaphrodites two individuals, either occasionallyor habitually, concur for the reproduction of their kind. This view waslong ago doubtfully suggested by Sprengel, Knight and Kolreuter. We shallpresently see its importance; but I must here treat the subject withextreme brevity, though I have the materials prepared for an amplediscussion. All vertebrate animals, all insects and some other largegroups of animals, pair for each birth. Modern research has muchdiminished the number of supposed hermaphrodites and of real hermaphroditesa large number pair; that is, two individuals regularly unite forreproduction, which is all that concerns us. But still there are manyhermaphrodite animals which certainly do not habitually pair, and a vastmajority of plants are hermaphrodites. What reason, it may be asked, isthere for supposing in these cases that two individuals ever concur inreproduction? As it is impossible here to enter on details, I must trustto some general considerations alone.

In the first place, I have collected so large a body of facts, and made somany experiments, showing, in accordance with the almost universal beliefof breeders, that with animals and plants a cross between differentvarieties, or between individuals of the same variety but of anotherstrain, gives vigour and fertility to the offspring; and on the other hand,that CLOSE interbreeding diminishes vigour and fertility; that these factsalone incline me to believe that it is a general law of nature that noorganic being fertilises itself for a perpetuity of generations; but that across with another individual is occasionally--perhaps at long intervals oftime--indispensable.

On the belief that this is a law of nature, we can, I think, understandseveral large classes of facts, such as the following, which on any otherview are inexplicable. Every hybridizer knows how unfavourable exposure towet is to the fertilisation of a flower, yet what a multitude of flowershave their anthers and stigmas fully exposed to the weather! If anoccasional cross be indispensable, notwithstanding that the plant's ownanthers and pistil stand so near each other as almost to ensure self-fertilisation, the fullest freedom for the entrance of pollen from anotherindividual will explain the above state of exposure of the organs. Manyflowers, on the other hand, have their organs of fructification closelyenclosed, as in the great papilionaceous or pea-family; but these almostinvariably present beautiful and curious adaptations in relation to thevisits of insects. So necessary are the visits of bees to manypapilionaceous flowers, that their fertility is greatly diminished if thesevisits be prevented. Now, it is scarcely possible for insects to fly fromflower to flower, and not to carry pollen from one to the other, to thegreat good of the plant. Insects act like a camel-hair pencil, and it issufficient, to ensure fertilisation, just to touch with the same brush theanthers of one flower and then the stigma of another; but it must not besupposed that bees would thus produce a multitude of hybrids betweendistinct species; for if a plant's own pollen and that from another speciesare placed on the same stigma, the former is so prepotent that itinvariably and completely destroys, as has been shown by Gartner, theinfluence of the foreign pollen.

When the stamens of a flower suddenly spring towards the pistil, or slowlymove one after the other towards it, the contrivance seems adapted solelyto ensure self-fertilisation; and no doubt it is useful for this end: butthe agency of insects is often required to cause the stamens to springforward, as Kolreuter has shown to be the case with the barberry; and inthis very genus, which seems to have a special contrivance forself-fertilisation, it is well known that, if closely-allied forms orvarieties are planted near each other, it is hardly possible to raise pureseedlings, so largely do they naturally cross. In numerous other cases,far from self-fertilisation being favoured, there are special contrivanceswhich effectually prevent the stigma receiving pollen from its own flower,as I could show from the works of Sprengel and others, as well as from myown observations: for instance, in Lobelia fulgens, there is a reallybeautiful and elaborate contrivance by which all the infinitely numerouspollen-granules are swept out of the conjoined anthers of each flower,before the stigma of that individual flower is ready to receive them; andas this flower is never visited, at least in my garden, by insects, itnever sets a seed, though by placing pollen from one flower on the stigmaof another, I raise plenty of seedlings. Another species of Lobelia, whichis visited by bees, seeds freely in my garden. In very many other cases,though there is no special mechanical contrivance to prevent the stigmareceiving pollen from the same flower, yet, as Sprengel, and more recentlyHildebrand and others have shown, and as I can confirm, either the anthersburst before the stigma is ready for fertilisation, or the stigma is readybefore the pollen of that flower is ready, so that these so-nameddichogamous plants have in fact separated sexes, and must habitually becrossed. So it is with the reciprocally dimorphic and trimorphic plantspreviously alluded to. How strange are these facts! How strange that thepollen and stigmatic surface of the same flower, though placed so closetogether, as if for the very purpose of self-fertilisation, should be in somany cases mutually useless to each other! How simply are these factsexplained on the view of an occasional cross with a distinct individualbeing advantageous or indispensable!

If several varieties of the cabbage, radish, onion, and of some otherplants, be allowed to seed near each other, a large majority of theseedlings thus raised turn out, as I found, mongrels: for instance, Iraised 233 seedling cabbages from some plants of different varietiesgrowing near each other, and of these only 78 were true to their kind, andsome even of these were not perfectly true. Yet the pistil of eachcabbage-flower is surrounded not only by its own six stamens but by thoseof the many other flowers on the same plant; and the pollen of each flowerreadily gets on its stigma without insect agency; for I have found thatplants carefully protected from insects produce the full number of pods. How, then, comes it that such a vast number of the seedlings aremongrelized? It must arise from the pollen of a distinct VARIETY having aprepotent effect over the flower's own pollen; and that this is part of thegeneral law of good being derived from the intercrossing of distinctindividuals of the same species. When distinct SPECIES are crossed thecase is reversed, for a plant's own pollen is always prepotent over foreignpollen; but to this subject we shall return in a future chapter.

In the case of a large tree covered with innumerable flowers, it may beobjected that pollen could seldom be carried from tree to tree, and at mostonly from flower to flower on the same tree; and flowers on the same treecan be considered as distinct individuals only in a limited sense. Ibelieve this objection to be valid, but that nature has largely providedagainst it by giving to trees a strong tendency to bear flowers withseparated sexes. When the sexes are separated, although the male andfemale flowers may be produced on the same tree, pollen must be regularlycarried from flower to flower; and this will give a better chance of pollenbeing occasionally carried from tree to tree. That trees belonging to allorders have their sexes more often separated than other plants, I find tobe the case in this country; and at my request Dr. Hooker tabulated thetrees of New Zealand, and Dr. Asa Gray those of the United States, and theresult was as I anticipated. On the other hand, Dr. Hooker informs me thatthe rule does not hold good in Australia: but if most of the Australiantrees are dichogamous, the same result would follow as if they bore flowerswith separated sexes. I have made these few remarks on trees simply tocall attention to the subject.

Turning for a brief space to animals: various terrestrial species arehermaphrodites, such as the land-mollusca and earth-worms; but these allpair. As yet I have not found a single terrestrial animal which canfertilise itself. This remarkable fact, which offers so strong a contrastwith terrestrial plants, is intelligible on the view of an occasional crossbeing indispensable; for owing to the nature of the fertilising elementthere are no means, analogous to the action of insects and of the wind withplants, by which an occasional cross could be effected with terrestrialanimals without the concurrence of two individuals. Of aquatic animals,there are many self-fertilising hermaphrodites; but here the currents ofwater offer an obvious means for an occasional cross. As in the case offlowers, I have as yet failed, after consultation with one of the highestauthorities, namely, Professor Huxley, to discover a single hermaphroditeanimal with the organs of reproduction so perfectly enclosed that accessfrom without, and the occasional influence of a distinct individual, can beshown to be physically impossible. Cirripedes long appeared to me topresent, under this point of view, a case of great difficulty; but I havebeen enabled, by a fortunate chance, to prove that two individuals, thoughboth are self-fertilising hermaphrodites, do sometimes cross.

It must have struck most naturalists as a strange anomaly that, both withanimals and plants, some species of the same family and even of the samegenus, though agreeing closely with each other in their whole organisation,are hermaphrodites, and some unisexual. But if, in fact, allhermaphrodites do occasionally intercross, the difference between them andunisexual species is, as far as function is concerned, very small.

>From these several considerations and from the many special facts which Ihave collected, but which I am unable here to give, it appears that withanimals and plants an occasional intercross between distinct individuals isa very general, if not universal, law of nature.

CIRCUMSTANCES FAVOURABLE FOR THE PRODUCTION OF NEW FORMS THROUGH NATURALSELECTION.

This is an extremely intricate subject. A great amount of variability,under which term individual differences are always included, will evidentlybe favourable. A large number of individuals, by giving a better chancewithin any given period for the appearance of profitable variations, willcompensate for a lesser amount of variability in each individual, and is, Ibelieve, a highly important element of success. Though nature grants longperiods of time for the work of natural selection, she does not grant anindefinite period; for as all organic beings are striving to seize on eachplace in the economy of nature, if any one species does not become modifiedand improved in a corresponding degree with its competitors it will beexterminated. Unless favourable variations be inherited by some at leastof the offspring, nothing can be effected by natural selection. Thetendency to reversion may often check or prevent the work; but as thistendency has not prevented man from forming by selection numerous domesticraces, why should it prevail against natural selection?

In the case of methodical selection, a breeder selects for some definiteobject, and if the individuals be allowed freely to intercross, his workwill completely fail. But when many men, without intending to alter thebreed, have a nearly common standard of perfection, and all try to procureand breed from the best animals, improvement surely but slowly follows fromthis unconscious process of selection, notwithstanding that there is noseparation of selected individuals. Thus it will be under nature; forwithin a confined area, with some place in the natural polity not perfectlyoccupied, all the individuals varying in the right direction, though indifferent degrees, will tend to be preserved. But if the area be large,its several districts will almost certainly present different conditions oflife; and then, if the same species undergoes modification in differentdistricts, the newly formed varieties will intercross on the confines ofeach. But we shall see in the sixth chapter that intermediate varieties,inhabiting intermediate districts, will in the long run generally besupplanted by one of the adjoining varieties. Intercrossing will chieflyaffect those animals which unite for each birth and wander much, and whichdo not breed at a very quick rate. Hence with animals of this nature, forinstance birds, varieties will generally be confined to separatedcountries; and this I find to be the case. With hermaphrodite organismswhich cross only occasionally, and likewise with animals which unite foreach birth, but which wander little and can increase at a rapid rate, a newand improved variety might be quickly formed on any one spot, and mightthere maintain itself in a body and afterward spread, so that theindividuals of the new variety would chiefly cross together. On thisprinciple nurserymen always prefer saving seed from a large body of plants,as the chance of intercrossing is thus lessened.

Even with animals which unite for each birth, and which do not propagaterapidly, we must not assume that free intercrossing would always eliminatethe effects of natural selection; for I can bring forward a considerablebody of facts showing that within the same area two varieties of the sameanimal may long remain distinct, from haunting different stations, frombreeding at slightly different seasons, or from the individuals of eachvariety preferring to pair together.

Intercrossing plays a very important part in nature by keeping theindividuals of the same species, or of the same variety, true and uniformin character. It will obviously thus act far more efficiently with thoseanimals which unite for each birth; but, as already stated, we have reasonto believe that occasional intercrosses take place with all animals andplants. Even if these take place only at long intervals of time, the youngthus produced will gain so much in vigour and fertility over the offspringfrom long-continued self-fertilisation, that they will have a better chanceof surviving and propagating their kind; and thus in the long run theinfluence of crosses, even at rare intervals, will be great. With respectto organic beings extremely low in the scale, which do not propagatesexually, nor conjugate, and which cannot possibly intercross, uniformityof character can be retained by them under the same conditions of life,only through the principle of inheritance, and through natural selectionwhich will destroy any individuals departing from the proper type. If theconditions of life change and the form undergoes modification, uniformityof character can be given to the modified offspring, solely by naturalselection preserving similar favourable variations.

Isolation also is an important element in the modification of speciesthrough natural selection. In a confined or isolated area, if not verylarge, the organic and inorganic conditions of life will generally bealmost uniform; so that natural selection will tend to modify all thevarying individuals of the same species in the same manner. Intercrossingwith the inhabitants of the surrounding districts, will also be thusprevented. Moritz Wagner has lately published an interesting essay on thissubject, and has shown that the service rendered by isolation in preventingcrosses between newly-formed varieties is probably greater even than Isupposed. But from reasons already assigned I can by no means agree withthis naturalist, that migration and isolation are necessary elements forthe formation of new species. The importance of isolation is likewisegreat in preventing, after any physical change in the conditions, such asof climate, elevation of the land, etc., the immigration of better adaptedorganisms; and thus new places in the natural economy of the district willbe left open to be filled up by the modification of the old inhabitants. Lastly, isolation will give time for a new variety to be improved at a slowrate; and this may sometimes be of much importance. If, however, anisolated area be very small, either from being surrounded by barriers, orfrom having very peculiar physical conditions, the total number of theinhabitants will be small; and this will retard the production of newspecies through natural selection, by decreasing the chances of favourablevariations arising.

The mere lapse of time by itself does nothing, either for or againstnatural selection. I state this because it has been erroneously assertedthat the element of time has been assumed by me to play an all-importantpart in modifying species, as if all the forms of life were necessarilyundergoing change through some innate law. Lapse of time is only so farimportant, and its importance in this respect is great, that it gives abetter chance of beneficial variations arising and of their being selected,accumulated, and fixed. It likewise tends to increase the direct action ofthe physical conditions of life, in relation to the constitution of eachorganism.

If we turn to nature to test the truth of these remarks, and look at anysmall isolated area, such as an oceanic island, although the number of thespecies inhabiting it is small, as we shall see in our chapter onGeographical Distribution; yet of these species a very large proportion areendemic,--that is, have been produced there and nowhere else in the world. Hence an oceanic island at first sight seems to have been highly favourablefor the production of new species. But we may thus deceive ourselves, forto ascertain whether a small isolated area, or a large open area like acontinent, has been most favourable for the production of new organicforms, we ought to make the comparison within equal times; and this we areincapable of doing.

Although isolation is of great importance in the production of new species,on the whole I am inclined to believe that largeness of area is still moreimportant, especially for the production of species which shall provecapable of enduring for a long period, and of spreading widely. Throughouta great and open area, not only will there be a better chance of favourablevariations, arising from the large number of individuals of the samespecies there supported, but the conditions of life are much more complexfrom the large number of already existing species; and if some of thesemany species become modified and improved, others will have to be improvedin a corresponding degree, or they will be exterminated. Each new form,also, as soon as it has been much improved, will be able to spread over theopen and continuous area, and will thus come into competition with manyother forms. Moreover, great areas, though now continuous, will often,owing to former oscillations of level, have existed in a broken condition,so that the good effects of isolation will generally, to a certain extent,have concurred. Finally, I conclude that, although small isolated areashave been in some respects highly favourable for the production of newspecies, yet that the course of modification will generally have been morerapid on large areas; and what is more important, that the new formsproduced on large areas, which already have been victorious over manycompetitors, will be those that will spread most widely, and will give riseto the greatest number of new varieties and species. They will thus play amore important part in the changing history of the organic world.

In accordance with this view, we can, perhaps, understand some facts whichwill be again alluded to in our chapter on Geographical Distribution; forinstance, the fact of the productions of the smaller continent of Australianow yielding before those of the larger Europaeo-Asiatic area. Thus, also,it is that continental productions have everywhere become so largelynaturalised on islands. On a small island, the race for life will havebeen less severe, and there will have been less modification and lessextermination. Hence, we can understand how it is that the flora ofMadeira, according to Oswald Heer, resembles to a certain extent theextinct tertiary flora of Europe. All fresh water basins, taken together,make a small area compared with that of the sea or of the land. Consequently, the competition between fresh water productions will havebeen less severe than elsewhere; new forms will have been more slowlyproduced, and old forms more slowly exterminated. And it is in fresh waterbasins that we find seven genera of Ganoid fishes, remnants of a oncepreponderant order: and in fresh water we find some of the most anomalousforms now known in the world, as the Ornithorhynchus and Lepidosiren,which, like fossils, connect to a certain extent orders at present widelyseparated in the natural scale. These anomalous forms may be called livingfossils; they have endured to the present day, from having inhabited aconfined area, and from having been exposed to less varied, and thereforeless severe, competition.

To sum up, as far as the extreme intricacy of the subject permits, thecircumstances favourable and unfavourable for the production of new speciesthrough natural selection. I conclude that for terrestrial productions alarge continental area, which has undergone many oscillations of level,will have been the most favourable for the production of many new forms oflife, fitted to endure for a long time and to spread widely. While thearea existed as a continent the inhabitants will have been numerous inindividuals and kinds, and will have been subjected to severe competition. When converted by subsidence into large separate islands there will stillhave existed many individuals of the same species on each island: intercrossing on the confines of the range of each new species will havebeen checked: after physical changes of any kind immigration will havebeen prevented, so that new places in the polity of each island will havehad to be filled up by the modification of the old inhabitants; and timewill have been allowed for the varieties in each to become well modifiedand perfected. When, by renewed elevation, the islands were reconvertedinto a continental area, there will again have been very severecompetition; the most favoured or improved varieties will have been enabledto spread; there will have been much extinction of the less improved forms,and the relative proportional numbers of the various inhabitants of thereunited continent will again have been changed; and again there will havebeen a fair field for natural selection to improve still further theinhabitants, and thus to produce new species.

That natural selection generally act with extreme slowness I fully admit. It can act only when there are places in the natural polity of a districtwhich can be better occupied by the modification of some of its existinginhabitants. The occurrence of such places will often depend on physicalchanges, which generally take place very slowly, and on the immigration ofbetter adapted forms being prevented. As some few of the old inhabitantsbecome modified the mutual relations of others will often be disturbed; andthis will create new places, ready to be filled up by better adapted forms;but all this will take place very slowly. Although all the individuals ofthe same species differ in some slight degree from each other, it wouldoften be long before differences of the right nature in various parts ofthe organisation might occur. The result would often be greatly retardedby free intercrossing. Many will exclaim that these several causes areamply sufficient to neutralise the power of natural selection. I do notbelieve so. But I do believe that natural selection will generally actvery slowly, only at long intervals of time, and only on a few of theinhabitants of the same region. I further believe that these slow,intermittent results accord well with what geology tells us of the rate andmanner at which the inhabitants of the world have changed.

Slow though the process of selection may be, if feeble man can do much byartificial selection, I can see no limit to the amount of change, to thebeauty and complexity of the coadaptations between all organic beings, onewith another and with their physical conditions of life, which may havebeen effected in the long course of time through nature's power ofselection, that is by the survival of the fittest.

EXTINCTION CAUSED BY NATURAL SELECTION.

This subject will be more fully discussed in our chapter on Geology; but itmust here be alluded to from being intimately connected with naturalselection. Natural selection acts solely through the preservation ofvariations in some way advantageous, which consequently endure. Owing tothe high geometrical rate of increase of all organic beings, each area isalready fully stocked with inhabitants, and it follows from this, that asthe favoured forms increase in number, so, generally, will the lessfavoured decrease and become rare. Rarity, as geology tells us, is theprecursor to extinction. We can see that any form which is represented byfew individuals will run a good chance of utter extinction, during greatfluctuations in the nature or the seasons, or from a temporary increase inthe number of its enemies. But we may go further than this; for as newforms are produced, unless we admit that specific forms can go onindefinitely increasing in number, many old forms must become extinct. That the number of specific forms has not indefinitely increased, geologyplainly tells us; and we shall presently attempt to show why it is that thenumber of species throughout the world has not become immeasurably great.

We have seen that the species which are most numerous in individuals havethe best chance of producing favourable variations within any given period. We have evidence of this, in the facts stated in the second chapter,showing that it is the common and diffused or dominant species which offerthe greatest number of recorded varieties. Hence, rare species will beless quickly modified or improved within any given period; they willconsequently be beaten in the race for life by the modified and improveddescendants of the commoner species.

>From these several considerations I think it inevitably follows, that asnew species in the course of time are formed through natural selection,others will become rarer and rarer, and finally extinct. The forms whichstand in closest competition with those undergoing modification andimprovement, will naturally suffer most. And we have seen in the chapteron the Struggle for Existence that it is the most closely-alliedforms,--varieties of the same species, and species of the same genus orrelated genera,--which, from having nearly the same structure, constitutionand habits, generally come into the severest competition with each other. Consequently, each new variety or species, during the progress of itsformation, will generally press hardest on its nearest kindred, and tend toexterminate them. We see the same process of extermination among ourdomesticated productions, through the selection of improved forms by man. Many curious instances could be given showing how quickly new breeds ofcattle, sheep and other animals, and varieties of flowers, take the placeof older and inferior kinds. In Yorkshire, it is historically known thatthe ancient black cattle were displaced by the long-horns, and that these"were swept away by the short-horns" (I quote the words of an agriculturalwriter) "as if by some murderous pestilence."

DIVERGENCE OF CHARACTER.

The principle, which I have designated by this term, is of high importance,and explains, as I believe, several important facts. In the first place,varieties, even strongly-marked ones, though having somewhat of thecharacter of species--as is shown by the hopeless doubts in many cases howto rank them--yet certainly differ far less from each other than do goodand distinct species. Nevertheless according to my view, varieties arespecies in the process of formation, or are, as I have called them,incipient species. How, then, does the lesser difference between varietiesbecome augmented into the greater difference between species? That thisdoes habitually happen, we must infer from most of the innumerable speciesthroughout nature presenting well-marked differences; whereas varieties,the supposed prototypes and parents of future well-marked species, presentslight and ill-defined differences. Mere chance, as we may call it, mightcause one variety to differ in some character from its parents, and theoffspring of this variety again to differ from its parent in the very samecharacter and in a greater degree; but this alone would never account forso habitual and large a degree of difference as that between the species ofthe same genus.

As has always been my practice, I have sought light on this head from ourdomestic productions. We shall here find something analogous. It will beadmitted that the production of races so different as short-horn andHereford cattle, race and cart horses, the several breeds of pigeons, etc.,could never have been effected by the mere chance accumulation of similarvariations during many successive generations. In practice, a fancier is,for instance, struck by a pigeon having a slightly shorter beak; anotherfancier is struck by a pigeon having a rather longer beak; and on theacknowledged principle that "fanciers do not and will not admire a mediumstandard, but like extremes," they both go on (as has actually occurredwith the sub-breeds of the tumbler-pigeon) choosing and breeding from birdswith longer and longer beaks, or with shorter and shorter beaks. Again, wemay suppose that at an early period of history, the men of one nation ordistrict required swifter horses, while those of another required strongerand bulkier horses. The early differences would be very slight; but, inthe course of time, from the continued selection of swifter horses in theone case, and of stronger ones in the other, the differences would becomegreater, and would be noted as forming two sub-breeds. Ultimately afterthe lapse of centuries, these sub-breeds would become converted into twowell-established and distinct breeds. As the differences became greater,the inferior animals with intermediate characters, being neither very swiftnor very strong, would not have been used for breeding, and will thus havetended to disappear. Here, then, we see in man's productions the action ofwhat may be called the principle of divergence, causing differences, atfirst barely appreciable, steadily to increase, and the breeds to divergein character, both from each other and from their common parent.

But how, it may be asked, can any analogous principle apply in nature? Ibelieve it can and does apply most efficiently (though it was a long timebefore I saw how), from the simple circumstance that the more diversifiedthe descendants from any one species become in structure, constitution, andhabits, by so much will they be better enabled to seize on many and widelydiversified places in the polity of nature, and so be enabled to increasein numbers.

We can clearly discern this in the case of animals with simple habits. Take the case of a carnivorous quadruped, of which the number that can besupported in any country has long ago arrived at its full average. If itsnatural power of increase be allowed to act, it can succeed in increasing(the country not undergoing any change in conditions) only by its varyingdescendants seizing on places at present occupied by other animals: someof them, for instance, being enabled to feed on new kinds of prey, eitherdead or alive; some inhabiting new stations, climbing trees, frequentingwater, and some perhaps becoming less carnivorous. The more diversified inhabits and structure the descendants of our carnivorous animals become, themore places they will be enabled to occupy. What applies to one animalwill apply throughout all time to all animals--that is, if they vary--forotherwise natural selection can effect nothing. So it will be with plants. It has been experimentally proved, that if a plot of ground be sown withone species of grass, and a similar plot be sown with several distinctgenera of grasses, a greater number of plants and a greater weight of dryherbage can be raised in the latter than in the former case. The same hasbeen found to hold good when one variety and several mixed varieties ofwheat have been sown on equal spaces of ground. Hence, if any one speciesof grass were to go on varying, and the varieties were continually selectedwhich differed from each other in the same manner, though in a very slightdegree, as do the distinct species and genera of grasses, a greater numberof individual plants of this species, including its modified descendants,would succeed in living on the same piece of ground. And we know that eachspecies and each variety of grass is annually sowing almost countlessseeds; and is thus striving, as it may be said, to the utmost to increasein number. Consequently, in the course of many thousand generations, themost distinct varieties of any one species of grass would have the bestchance of succeeding and of increasing in numbers, and thus of supplantingthe less distinct varieties; and varieties, when rendered very distinctfrom each other, take the rank of species.

The truth of the principle that the greatest amount of life can besupported by great diversification of structure, is seen under many naturalcircumstances. In an extremely small area, especially if freely open toimmigration, and where the contest between individual and individual mustbe very severe, we always find great diversity in its inhabitants. Forinstance, I found that a piece of turf, three feet by four in size, whichhad been exposed for many years to exactly the same conditions, supportedtwenty species of plants, and these belonged to eighteen genera and toeight orders, which shows how much these plants differed from each other. So it is with the plants and insects on small and uniform islets: also insmall ponds of fresh water. Farmers find that they can raise more food bya rotation of plants belonging to the most different orders: naturefollows what may be called a simultaneous rotation. Most of the animalsand plants which live close round any small piece of ground, could live onit (supposing its nature not to be in any way peculiar), and may be said tobe striving to the utmost to live there; but, it is seen, that where theycome into the closest competition, the advantages of diversification ofstructure, with the accompanying differences of habit and constitution,determine that the inhabitants, which thus jostle each other most closely,shall, as a general rule, belong to what we call different genera andorders.

The same principle is seen in the naturalisation of plants through man'sagency in foreign lands. It might have been expected that the plants whichwould succeed in becoming naturalised in any land would generally have beenclosely allied to the indigenes; for these are commonly looked at asspecially created and adapted for their own country. It might also,perhaps, have been expected that naturalised plants would have belonged toa few groups more especially adapted to certain stations in their newhomes. But the case is very different; and Alph. de Candolle has wellremarked, in his great and admirable work, that floras gain bynaturalisation, proportionally with the number of the native genera andspecies, far more in new genera than in new species. To give a singleinstance: in the last edition of Dr. Asa Gray's "Manual of the Flora ofthe Northern United States," 260 naturalised plants are enumerated, andthese belong to 162 genera. We thus see that these naturalised plants areof a highly diversified nature. They differ, moreover, to a large extent,from the indigenes, for out of the 162 naturalised genera, no less than 100genera are not there indigenous, and thus a large proportional addition ismade to the genera now living in the United States.

By considering the nature of the plants or animals which have in anycountry struggled successfully with the indigenes, and have there becomenaturalised, we may gain some crude idea in what manner some of the nativeswould have had to be modified in order to gain an advantage over theircompatriots; and we may at least infer that diversification of structure,amounting to new generic differences, would be profitable to them.

The advantage of diversification of structure in the inhabitants of thesame region is, in fact, the same as that of the physiological division oflabour in the organs of the same individual body--a subject so wellelucidated by Milne Edwards. No physiologist doubts that a stomach bybeing adapted to digest vegetable matter alone, or flesh alone, draws mostnutriment from these substances. So in the general economy of any land,the more widely and perfectly the animals and plants are diversified fordifferent habits of life, so will a greater number of individuals becapable of there supporting themselves. A set of animals, with theirorganisation but little diversified, could hardly compete with a set moreperfectly diversified in structure. It may be doubted, for instance,whether the Australian marsupials, which are divided into groups differingbut little from each other, and feebly representing, as Mr. Waterhouse andothers have remarked, our carnivorous, ruminant, and rodent mammals, couldsuccessfully compete with these well-developed orders. In the Australianmammals, we see the process of diversification in an early and incompletestage of development.

THE PROBABLE EFFECTS OF THE ACTION OF NATURAL SELECTION THROUGH DIVERGENCEOF CHARACTER AND EXTINCTION, ON THE DESCENDANTS OF A COMMON ANCESTOR.

After the foregoing discussion, which has been much compressed, we mayassume that the modified descendants of any one species will succeed somuch the better as they become more diversified in structure, and are thusenabled to encroach on places occupied by other beings. Now let us see howthis principle of benefit being derived from divergence of character,combined with the principles of natural selection and of extinction, tendsto act.

The accompanying diagram will aid us in understanding this ratherperplexing subject. Let A to L represent the species of a genus large inits own country; these species are supposed to resemble each other inunequal degrees, as is so generally the case in nature, and as isrepresented in the diagram by the letters standing at unequal distances. Ihave said a large genus, because as we saw in the second chapter, on anaverage more species vary in large genera than in small genera; and thevarying species of the large genera present a greater number of varieties. We have, also, seen that the species, which are the commonest and mostwidely-diffused, vary more than do the rare and restricted species. Let(A) be a common, widely-diffused, and varying species, belonging to a genuslarge in its own country. The branching and diverging dotted lines ofunequal lengths proceeding from (A), may represent its varying offspring. The variations are supposed to be extremely slight, but of the mostdiversified nature; they are not supposed all to appear simultaneously, butoften after long intervals of time; nor are they all supposed to endure forequal periods. Only those variations which are in some way profitable willbe preserved or naturally selected. And here the importance of theprinciple of benefit derived from divergence of character comes in; forthis will generally lead to the most different or divergent variations(represented by the outer dotted lines) being preserved and accumulated bynatural selection. When a dotted line reaches one of the horizontal lines,and is there marked by a small numbered letter, a sufficient amount ofvariation is supposed to have been accumulated to form it into a fairlywell-marked variety, such as would be thought worthy of record in asystematic work.

The intervals between the horizontal lines in the diagram, may representeach a thousand or more generations. After a thousand generations, species(A) is supposed to have produced two fairly well-marked varieties, namelya1 and m1. These two varieties will generally still be exposed to the sameconditions which made their parents variable, and the tendency tovariability is in itself hereditary; consequently they will likewise tendto vary, and commonly in nearly the same manner as did their parents. Moreover, these two varieties, being only slightly modified forms, willtend to inherit those advantages which made their parent (A) more numerousthan most of the other inhabitants of the same country; they will alsopartake of those more general advantages which made the genus to which theparent-species belonged, a large genus in its own country. And all thesecircumstances are favourable to the production of new varieties.

If, then, these two varieties be variable, the most divergent of theirvariations will generally be preserved during the next thousandgenerations. And after this interval, variety a1 is supposed in thediagram to have produced variety a2, which will, owing to the principle ofdivergence, differ more from (A) than did variety a1. Variety m1 issupposed to have produced two varieties, namely m2 and s2, differing fromeach other, and more considerably from their common parent (A). We maycontinue the process by similar steps for any length of time; some of thevarieties, after each thousand generations, producing only a singlevariety, but in a more and more modified condition, some producing two orthree varieties, and some failing to produce any. Thus the varieties ormodified descendants of the common parent (A), will generally go onincreasing in number and diverging in character. In the diagram theprocess is represented up to the ten-thousandth generation, and under acondensed and simplified form up to the fourteen-thousandth generation.

But I must here remark that I do not suppose that the process ever goes onso regularly as is represented in the diagram, though in itself madesomewhat irregular, nor that it goes on continuously; it is far moreprobable that each form remains for long periods unaltered, and then againundergoes modification. Nor do I suppose that the most divergent varietiesare invariably preserved: a medium form may often long endure, and may ormay not produce more than one modified descendant; for natural selectionwill always act according to the nature of the places which are eitherunoccupied or not perfectly occupied by other beings; and this will dependon infinitely complex relations. But as a general rule, the morediversified in structure the descendants from any one species can berendered, the more places they will be enabled to seize on, and the moretheir modified progeny will increase. In our diagram the line ofsuccession is broken at regular intervals by small numbered letters markingthe successive forms which have become sufficiently distinct to be recordedas varieties. But these breaks are imaginary, and might have been insertedanywhere, after intervals long enough to allow the accumulation of aconsiderable amount of divergent variation.

As all the modified descendants from a common and widely-diffused species,belonging to a large genus, will tend to partake of the same advantageswhich made their parent successful in life, they will generally go onmultiplying in number as well as diverging in character: this isrepresented in the diagram by the several divergent branches proceedingfrom (A). The modified offspring from the later and more highly improvedbranches in the lines of descent, will, it is probable, often take theplace of, and so destroy, the earlier and less improved branches: this isrepresented in the diagram by some of the lower branches not reaching tothe upper horizontal lines. In some cases no doubt the process ofmodification will be confined to a single line of descent, and the numberof modified descendants will not be increased; although the amount ofdivergent modification may have been augmented. This case would berepresented in the diagram, if all the lines proceeding from (A) wereremoved, excepting that from a1 to a10. In the same way the Englishracehorse and English pointer have apparently both gone on slowly divergingin character from their original stocks, without either having given offany fresh branches or races.

After ten thousand generations, species (A) is supposed to have producedthree forms, a10, f10, and m10, which, from having diverged in characterduring the successive generations, will have come to differ largely, butperhaps unequally, from each other and from their common parent. If wesuppose the amount of change between each horizontal line in our diagram tobe excessively small, these three forms may still be only well-markedvarieties; but we have only to suppose the steps in the process ofmodification to be more numerous or greater in amount, to convert thesethree forms into doubtful or at least into well-defined species: thus thediagram illustrates the steps by which the small differences distinguishingvarieties are increased into the larger differences distinguishing species. By continuing the same process for a greater number of generations (asshown in the diagram in a condensed and simplified manner), we get eightspecies, marked by the letters between a14 and m14, all descended from (A). Thus, as I believe, species are multiplied and genera are formed.

In a large genus it is probable that more than one species would vary. Inthe diagram I have assumed that a second species (I) has produced, byanalogous steps, after ten thousand generations, either two well-markedvarieties (w10 and z10) or two species, according to the amount of changesupposed to be represented between the horizontal lines. After fourteenthousand generations, six new species, marked by the letters n14 to z14,are supposed to have been produced. In any genus, the species which arealready very different in character from each other, will generally tend toproduce the greatest number of modified descendants; for these will havethe best chance of seizing on new and widely different places in the polityof nature: hence in the diagram I have chosen the extreme species (A), andthe nearly extreme species (I), as those which have largely varied, andhave given rise to new varieties and species. The other nine species(marked by capital letters) of our original genus, may for long but unequalperiods continue to transmit unaltered descendants; and this is shown inthe diagram by the dotted lines unequally prolonged upwards.

But during the process of modification, represented in the diagram, anotherof our principles, namely that of extinction, will have played an importantpart. As in each fully stocked country natural selection necessarily actsby the selected form having some advantage in the struggle for life overother forms, there will be a constant tendency in the improved descendantsof any one species to supplant and exterminate in each stage of descenttheir predecessors and their original progenitor. For it should beremembered that the competition will generally be most severe between thoseforms which are most nearly related to each other in habits, constitutionand structure. Hence all the intermediate forms between the earlier andlater states, that is between the less and more improved states of a thesame species, as well as the original parent-species itself, will generallytend to become extinct. So it probably will be with many whole collaterallines of descent, which will be conquered by later and improved lines. If,however, the modified offspring of a species get into some distinctcountry, or become quickly adapted to some quite new station, in whichoffspring and progenitor do not come into competition, both may continue toexist.

If, then, our diagram be assumed to represent a considerable amount ofmodification, species (A) and all the earlier varieties will have becomeextinct, being replaced by eight new species (a14 to m14); and species (I)will be replaced by six (n14 to z14) new species.

But we may go further than this. The original species of our genus weresupposed to resemble each other in unequal degrees, as is so generally thecase in nature; species (A) being more nearly related to B, C, and D thanto the other species; and species (I) more to G, H, K, L, than to theothers. These two species (A and I), were also supposed to be very commonand widely diffused species, so that they must originally have had someadvantage over most of the other species of the genus. Their modifieddescendants, fourteen in number at the fourteen-thousandth generation, willprobably have inherited some of the same advantages: they have also beenmodified and improved in a diversified manner at each stage of descent, soas to have become adapted to many related places in the natural economy oftheir country. It seems, therefore, extremely probable that they will havetaken the places of, and thus exterminated, not only their parents (A) and(I), but likewise some of the original species which were most nearlyrelated to their parents. Hence very few of the original species will havetransmitted offspring to the fourteen-thousandth generation. We maysuppose that only one (F) of the two species (E and F) which were leastclosely related to the other nine original species, has transmitteddescendants to this late stage of descent.

The new species in our diagram, descended from the original eleven species,will now be fifteen in number. Owing to the divergent tendency of naturalselection, the extreme amount of difference in character between speciesa14 and z14 will be much greater than that between the most distinct of theoriginal eleven species. The new species, moreover, will be allied to eachother in a widely different manner. Of the eight descendants from (A) thethree marked a14, q14, p14, will be nearly related from having recentlybranched off from a10; b14 and f14, from having diverged at an earlierperiod from a5, will be in some degree distinct from the three first-namedspecies; and lastly, o14, e14, and m14, will be nearly related one to theother, but, from having diverged at the first commencement of the processof modification, will be widely different from the other five species, andmay constitute a sub-genus or a distinct genus.

The six descendants from (I) will form two sub-genera or genera. But asthe original species (I) differed largely from (A), standing nearly at theextreme end of the original genus, the six descendants from (I) will, owingto inheritance alone, differ considerably from the eight descendants from(A); the two groups, moreover, are supposed to have gone on diverging indifferent directions. The intermediate species, also (and this is a veryimportant consideration), which connected the original species (A) and (I),have all become, except (F), extinct, and have left no descendants. Hencethe six new species descended from (I), and the eight descendants from (A),will have to be ranked as very distinct genera, or even as distinctsub-families.

Thus it is, as I believe, that two or more genera are produced by descentwith modification, from two or more species of the same genus. And the twoor more parent-species are supposed to be descended from some one speciesof an earlier genus. In our diagram this is indicated by the broken linesbeneath the capital letters, converging in sub-branches downwards towards asingle point; this point represents a species, the supposed progenitor ofour several new sub-genera and genera.

It is worth while to reflect for a moment on the character of the newspecies F14, which is supposed not to have diverged much in character, butto have retained the form of (F), either unaltered or altered only in aslight degree. In this case its affinities to the other fourteen newspecies will be of a curious and circuitous nature. Being descended from aform that stood between the parent-species (A) and (I), now supposed to beextinct and unknown, it will be in some degree intermediate in characterbetween the two groups descended from these two species. But as these twogroups have gone on diverging in character from the type of their parents,the new species (F14) will not be directly intermediate between them, butrather between types of the two groups; and every naturalist will be ableto call such cases before his mind.

In the diagram each horizontal line has hitherto been supposed to representa thousand generations, but each may represent a million or moregenerations; it may also represent a section of the successive strata ofthe earth's crust including extinct remains. We shall, when we come to ourchapter on geology, have to refer again to this subject, and I think weshall then see that the diagram throws light on the affinities of extinctbeings, which, though generally belonging to the same orders, families, orgenera, with those now living, yet are often, in some degree, intermediatein character between existing groups; and we can understand this fact, forthe extinct species lived at various remote epochs when the branching linesof descent had diverged less.

I see no reason to limit the process of modification, as now explained, tothe formation of genera alone. If, in the diagram, we suppose the amountof change represented by each successive group of diverging dotted lines tobe great, the forms marked a14 to p14, those marked b14 and f14, and thosemarked o14 to m14, will form three very distinct genera. We shall alsohave two very distinct genera descended from (I), differing widely from thedescendants of (A). These two groups of genera will thus form two distinctfamilies, or orders, according to the amount of divergent modificationsupposed to be represented in the diagram. And the two new families, ororders, are descended from two species of the original genus; and these aresupposed to be descended from some still more ancient and unknown form.

We have seen that in each country it is the species belonging to the largergenera which oftenest present varieties or incipient species. This,indeed, might have been expected; for as natural selection acts through oneform having some advantage over other forms in the struggle for existence,it will chiefly act on those which already have some advantage; and thelargeness of any group shows that its species have inherited from a commonancestor some advantage in common. Hence, the struggle for the productionof new and modified descendants will mainly lie between the larger groups,which are all trying to increase in number. One large group will slowlyconquer another large group, reduce its number, and thus lessen its chanceof further variation and improvement. Within the same large group, thelater and more highly perfected sub-groups, from branching out and seizingon many new places in the polity of nature, will constantly tend tosupplant and destroy the earlier and less improved sub-groups. Small andbroken groups and sub-groups will finally disappear. Looking to thefuture, we can predict that the groups of organic beings which are nowlarge and triumphant, and which are least broken up, that is, which have asyet suffered least extinction, will, for a long period, continue toincrease. But which groups will ultimately prevail, no man can predict;for we know that many groups, formerly most extensively developed, have nowbecome extinct. Looking still more remotely to the future, we may predictthat, owing to the continued and steady increase of the larger groups, amultitude of smaller groups will become utterly extinct, and leave nomodified descendants; and consequently that, of the species living at anyone period, extremely few will transmit descendants to a remote futurity. I shall have to return to this subject in the chapter on classification,but I may add that as, according to this view, extremely few of the moreancient species have transmitted descendants to the present day, and, asall the descendants of the same species form a class, we can understand howit is that there exist so few classes in each main division of the animaland vegetable kingdoms. Although few of the most ancient species have leftmodified descendants, yet, at remote geological periods, the earth may havebeen almost as well peopled with species of many genera, families, ordersand classes, as at the present day.

ON THE DEGREE TO WHICH ORGANISATION TENDS TO ADVANCE.

Natural selection acts exclusively by the preservation and accumulation ofvariations, which are beneficial under the organic and inorganic conditionsto which each creature is exposed at all periods of life. The ultimateresult is that each creature tends to become more and more improved inrelation to its conditions. This improvement inevitably leads to thegradual advancement of the organisation of the greater number of livingbeings throughout the world. But here we enter on a very intricatesubject, for naturalists have not defined to each other's satisfaction whatis meant by an advance in organisation. Among the vertebrata the degree ofintellect and an approach in structure to man clearly come into play. Itmight be thought that the amount of change which the various parts andorgans pass through in their development from embryo to maturity wouldsuffice as a standard of comparison; but there are cases, as with certainparasitic crustaceans, in which several parts of the structure become lessperfect, so that the mature animal cannot be called higher than its larva. Von Baer's standard seems the most widely applicable and the best, namely,the amount of differentiation of the parts of the same organic being, inthe adult state, as I should be inclined to add, and their specialisationfor different functions; or, as Milne Edwards would express it, thecompleteness of the division of physiological labour. But we shall see howobscure this subject is if we look, for instance, to fishes, among whichsome naturalists rank those as highest which, like the sharks, approachnearest to amphibians; while other naturalists rank the common bony orteleostean fishes as the highest, inasmuch as they are most strictly fish-like, and differ most from the other vertebrate classes. We see still moreplainly the obscurity of the subject by turning to plants, among which thestandard of intellect is of course quite excluded; and here some botanistsrank those plants as highest which have every organ, as sepals, petals,stamens and pistils, fully developed in each flower; whereas otherbotanists, probably with more truth, look at the plants which have theirseveral organs much modified and reduced in number as the highest.

If we take as the standard of high organisation, the amount ofdifferentiation and specialisation of the several organs in each being whenadult (and this will include the advancement of the brain for intellectualpurposes), natural selection clearly leads towards this standard: for allphysiologists admit that the specialisation of organs, inasmuch as in thisstate they perform their functions better, is an advantage to each being;and hence the accumulation of variations tending towards specialisation iswithin the scope of natural selection. On the other hand, we can see,bearing in mind that all organic beings are striving to increase at a highratio and to seize on every unoccupied or less well occupied place in theeconomy of nature, that it is quite possible for natural selectiongradually to fit a being to a situation in which several organs would besuperfluous or useless: in such cases there would be retrogression in thescale of organisation. Whether organisation on the whole has actuallyadvanced from the remotest geological periods to the present day will bemore conveniently discussed in our chapter on Geological Succession.

But it may be objected that if all organic beings thus tend to rise in thescale, how is it that throughout the world a multitude of the lowest formsstill exist; and how is it that in each great class some forms are far morehighly developed than others? Why have not the more highly developed formsevery where supplanted and exterminated the lower? Lamarck, who believedin an innate and inevitable tendency towards perfection in all organicbeings, seems to have felt this difficulty so strongly that he was led tosuppose that new and simple forms are continually being produced byspontaneous generation. Science has not as yet proved the truth of thisbelief, whatever the future may reveal. On our theory the continuedexistence of lowly organisms offers no difficulty; for natural selection,or the survival of the fittest, does not necessarily include progressivedevelopment--it only takes advantage of such variations as arise and arebeneficial to each creature under its complex relations of life. And itmay be asked what advantage, as far as we can see, would it be to aninfusorian animalcule--to an intestinal worm--or even to an earth-worm, tobe highly organised. If it were no advantage, these forms would be left,by natural selection, unimproved or but little improved, and might remainfor indefinite ages in their present lowly condition. And geology tells usthat some of the lowest forms, as the infusoria and rhizopods, haveremained for an enormous period in nearly their present state. But tosuppose that most of the many now existing low forms have not in the leastadvanced since the first dawn of life would be extremely rash; for everynaturalist who has dissected some of the beings now ranked as very low inthe scale, must have been struck with their really wondrous and beautifulorganisation.

Nearly the same remarks are applicable, if we look to the different gradesof organisation within the same great group; for instance, in thevertebrata, to the co-existence of mammals and fish--among mammalia, to theco-existence of man and the ornithorhynchus--among fishes, to the co-existence of the shark and the lancelet (Amphioxus), which latter fish inthe extreme simplicity of its structure approaches the invertebrateclasses. But mammals and fish hardly come into competition with eachother; the advancement of the whole class of mammals, or of certain membersin this class, to the highest grade would not lead to their taking theplace of fishes. Physiologists believe that the brain must be bathed bywarm blood to be highly active, and this requires aerial respiration; sothat warm-blooded mammals when inhabiting the water lie under adisadvantage in having to come continually to the surface to breathe. Withfishes, members of the shark family would not tend to supplant thelancelet; for the lancelet, as I hear from Fritz Muller, has as solecompanion and competitor on the barren sandy shore of South Brazil, ananomalous annelid. The three lowest orders of mammals, namely, marsupials,edentata, and rodents, co-exist in South America in the same region withnumerous monkeys, and probably interfere little with each other. Althoughorganisation, on the whole, may have advanced and be still advancingthroughout the world, yet the scale will always present many degrees ofperfection; for the high advancement of certain whole classes, or ofcertain members of each class, does not at all necessarily lead to theextinction of those groups with which they do not enter into closecompetition. In some cases, as we shall hereafter see, lowly organisedforms appear to have been preserved to the present day, from inhabitingconfined or peculiar stations, where they have been subjected to lesssevere competition, and where their scanty numbers have retarded the chanceof favourable variations arising.

Finally, I believe that many lowly organised forms now exist throughout theworld, from various causes. In some cases variations or individualdifferences of a favourable nature may never have arisen for naturalselection to act on and accumulate. In no case, probably, has timesufficed for the utmost possible amount of development. In some few casesthere has been what we must call retrogression or organisation. But themain cause lies in the fact that under very simple conditions of life ahigh organisation would be of no service--possibly would be of actualdisservice, as being of a more delicate nature, and more liable to be putout of order and injured.

Looking to the first dawn of life, when all organic beings, as we maybelieve, presented the simplest structure, how, it has been asked, couldthe first step in the advancement or differentiation of parts have arisen? Mr. Herbert Spencer would probably answer that, as soon as simpleunicellular organisms came by growth or division to be compounded ofseveral cells, or became attached to any supporting surface, his law "thathomologous units of any order become differentiated in proportion as theirrelations to incident forces become different" would come into action. Butas we have no facts to guide us, speculation on the subject is almostuseless. It is, however, an error to suppose that there would be nostruggle for existence, and, consequently, no natural selection, until manyforms had been produced: variations in a single species inhabiting anisolated station might be beneficial, and thus the whole mass ofindividuals might be modified, or two distinct forms might arise. But, asI remarked towards the close of the introduction, no one ought to feelsurprise at much remaining as yet unexplained on the origin of species, ifwe make due allowance for our profound ignorance on the mutual relations ofthe inhabitants of the world at the present time, and still more so duringpast ages.

CONVERGENCE OF CHARACTER.

Mr. H.C. Watson thinks that I have overrated the importance of divergenceof character (in which, however, he apparently believes), and thatconvergence, as it may be called, has likewise played a part. If twospecies belonging to two distinct though allied genera, had both produced alarge number of new and divergent forms, it is conceivable that these mightapproach each other so closely that they would have all to be classed underthe same genus; and thus the descendants of two distinct genera wouldconverge into one. But it would in most cases be extremely rash toattribute to convergence a close and general similarity of structure in themodified descendants of widely distinct forms. The shape of a crystal isdetermined solely by the molecular forces, and it is not surprising thatdissimilar substances should sometimes assume the same form; but withorganic beings we should bear in mind that the form of each depends on aninfinitude of complex relations, namely on the variations which havearisen, these being due to causes far too intricate to be followed out--onthe nature of the variations which have been preserved or selected, andthis depends on the surrounding physical conditions, and in a still higherdegree on the surrounding organisms with which each being has come intocompetition--and lastly, on inheritance (in itself a fluctuating element)from innumerable progenitors, all of which have had their forms determinedthrough equally complex relations. It is incredible that the descendantsof two organisms, which had originally differed in a marked manner, shouldever afterwards converge so closely as to lead to a near approach toidentity throughout their whole organisation. If this had occurred, weshould meet with the same form, independently of genetic connection,recurring in widely separated geological formations; and the balance ofevidence is opposed to any such an admission.

Mr. Watson has also objected that the continued action of naturalselection, together with divergence of character, would tend to make anindefinite number of specific forms. As far as mere inorganic conditionsare concerned, it seems probable that a sufficient number of species wouldsoon become adapted to all considerable diversities of heat, moisture,etc.; but I fully admit that the mutual relations of organic beings aremore important; and as the number of species in any country goes onincreasing, the organic conditions of life must become more and morecomplex. Consequently there seems at first no limit to the amount ofprofitable diversification of structure, and therefore no limit to thenumber of species which might be produced. We do not know that even themost prolific area is fully stocked with specific forms: at the Cape ofGood Hope and in Australia, which support such an astonishing number ofspecies, many European plants have become naturalised. But geology showsus, that from an early part of the tertiary period the number of species ofshells, and that from the middle part of this same period, the number ofmammals has not greatly or at all increased. What then checks anindefinite increase in the number of species? The amount of life (I do notmean the number of specific forms) supported on an area must have a limit,depending so largely as it does on physical conditions; therefore, if anarea be inhabited by very many species, each or nearly each species will berepresented by few individuals; and such species will be liable toextermination from accidental fluctuations in the nature of the seasons orin the number of their enemies. The process of extermination in such caseswould be rapid, whereas the production of new species must always be slow.Imagine the extreme case of as many species as individuals in England, andthe first severe winter or very dry summer would exterminate thousands onthousands of species. Rare species, and each species will become rare ifthe number of species in any country becomes indefinitely increased, will,on the principal often explained, present within a given period fewfavourable variations; consequently, the process of giving birth to newspecific forms would thus be retarded. When any species becomes very rare,close interbreeding will help to exterminate it; authors have thought thatthis comes into play in accounting for the deterioration of the aurochs inLithuania, of red deer in Scotland and of bears in Norway, etc. Lastly,and this I am inclined to think is the most important element, a dominantspecies, which has already beaten many competitors in its own home, willtend to spread and supplant many others. Alph. de Candolle has shown thatthose species which spread widely tend generally to spread VERY widely,consequently they will tend to supplant and exterminate several species inseveral areas, and thus check the inordinate increase of specific formsthroughout the world. Dr. Hooker has recently shown that in the southeastcorner of Australia, where, apparently, there are many invaders fromdifferent quarters of the globe, the endemic Australian species have beengreatly reduced in number. How much weight to attribute to these severalconsiderations I will not pretend to say; but conjointly they must limit ineach country the tendency to an indefinite augmentation of specific forms.

SUMMARY OF CHAPTER.

If under changing conditions of life organic beings present individualdifferences in almost every part of their structure, and this cannot bedisputed; if there be, owing to their geometrical rate of increase, asevere struggle for life at some age, season or year, and this certainlycannot be disputed; then, considering the infinite complexity of therelations of all organic beings to each other and to their conditions oflife, causing an infinite diversity in structure, constitution, and habits,to be advantageous to them, it would be a most extraordinary fact if novariations had ever occurred useful to each being's own welfare, in thesame manner as so many variations have occurred useful to man. But ifvariations useful to any organic being ever do occur, assuredly individualsthus characterised will have the best chance of being preserved in thestruggle for life; and from the strong principle of inheritance, these willtend to produce offspring similarly characterised. This principle ofpreservation, or the survival of the fittest, I have called naturalselection. It leads to the improvement of each creature in relation to itsorganic and inorganic conditions of life; and consequently, in most cases,to what must be regarded as an advance in organisation. Nevertheless, lowand simple forms will long endure if well fitted for their simpleconditions of life.

Natural selection, on the principle of qualities being inherited atcorresponding ages, can modify the egg, seed, or young as easily as theadult. Among many animals sexual selection will have given its aid toordinary selection by assuring to the most vigorous and best adapted malesthe greatest number of offspring. Sexual selection will also givecharacters useful to the males alone in their struggles or rivalry withother males; and these characters will be transmitted to one sex or to bothsexes, according to the form of inheritance which prevails.

Whether natural selection has really thus acted in adapting the variousforms of life to their several conditions and stations, must be judged bythe general tenour and balance of evidence given in the following chapters. But we have already seen how it entails extinction; and how largelyextinction has acted in the world's history, geology plainly declares. Natural selection, also, leads to divergence of character; for the moreorganic beings diverge in structure, habits and constitution, by so muchthe more can a large number be supported on the area, of which we see proofby looking to the inhabitants of any small spot, and to the productionsnaturalised in foreign lands. Therefore, during the modification of thedescendants of any one species, and during the incessant struggle of allspecies to increase in numbers, the more diversified the descendantsbecome, the better will be their chance of success in the battle for life. Thus the small differences distinguishing varieties of the same species,steadily tend to increase, till they equal the greater differences betweenspecies of the same genus, or even of distinct genera.

We have seen that it is the common, the widely diffused, and widely rangingspecies, belonging to the larger genera within each class, which vary most;and these tend to transmit to their modified offspring that superioritywhich now makes them dominant in their own countries. Natural selection,as has just been remarked, leads to divergence of character and to muchextinction of the less improved and intermediate forms of life. On theseprinciples, the nature of the affinities, and the generally well defineddistinctions between the innumerable organic beings in each classthroughout the world, may be explained. It is a truly wonderful fact--thewonder of which we are apt to overlook from familiarity--that all animalsand all plants throughout all time and space should be related to eachother in groups, subordinate to groups, in the manner which we everywherebehold--namely, varieties of the same species most closely related, speciesof the same genus less closely and unequally related, forming sections andsub-genera, species of distinct genera much less closely related, andgenera related in different degrees, forming sub-families, families,orders, sub-classes, and classes. The several subordinate groups in anyclass cannot be ranked in a single file, but seem clustered round points,and these round other points, and so on in almost endless cycles. Ifspecies had been independently created, no explanation would have beenpossible of this kind of classification; but it is explained throughinheritance and the complex action of natural selection, entailingextinction and divergence of character, as we have seen illustrated in thediagram.

The affinities of all the beings of the same class have sometimes beenrepresented by a great tree. I believe this simile largely speaks thetruth. The green and budding twigs may represent existing species; andthose produced during former years may represent the long succession ofextinct species. At each period of growth all the growing twigs have triedto branch out on all sides, and to overtop and kill the surrounding twigsand branches, in the same manner as species and groups of species have atall times overmastered other species in the great battle for life. Thelimbs divided into great branches, and these into lesser and lesserbranches, were themselves once, when the tree was young, budding twigs; andthis connexion of the former and present buds by ramifying branches maywell represent the classification of all extinct and living species ingroups subordinate to groups. Of the many twigs which flourished when thetree was a mere bush, only two or three, now grown into great branches, yetsurvive and bear the other branches; so with the species which lived duringlong-past geological periods, very few have left living and modifieddescendants. From the first growth of the tree, many a limb and branch hasdecayed and dropped off; and these fallen branches of various sizes mayrepresent those whole orders, families, and genera which have now no livingrepresentatives, and which are known to us only in a fossil state. As wehere and there see a thin, straggling branch springing from a fork low downin a tree, and which by some chance has been favoured and is still alive onits summit, so we occasionally see an animal like the Ornithorhynchus orLepidosiren, which in some small degree connects by its affinities twolarge branches of life, and which has apparently been saved from fatalcompetition by having inhabited a protected station. As buds give rise bygrowth to fresh buds, and these, if vigorous, branch out and overtop on allsides many a feebler branch, so by generation I believe it has been withthe great Tree of Life, which fills with its dead and broken branches thecrust of the earth, and covers the surface with its ever-branching andbeautiful ramifications.