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Difficulties of the theory of descent with modification -- Absence orrarity of transitional varieties -- Transitions in habits of life --Diversified habits in the same species -- Species with habits widelydifferent from those of their allies -- Organs of extreme perfection --Modes of transition -- Cases of difficulty -- Natura non facit saltum --Organs of small importance -- Organs not in all cases absolutely perfect --The law of Unity of Type and of the Conditions of Existence embraced by thetheory of Natural Selection.

Long before the reader has arrived at this part of my work, a crowd ofdifficulties will have occurred to him. Some of them are so serious thatto this day I can hardly reflect on them without being in some degreestaggered; but, to the best of my judgment, the greater number are onlyapparent, and those that are real are not, I think, fatal to the theory.

These difficulties and objections may be classed under the following heads: First, why, if species have descended from other species by finegradations, do we not everywhere see innumerable transitional forms? Whyis not all nature in confusion, instead of the species being, as we seethem, well defined?

Secondly, is it possible that an animal having, for instance, the structureand habits of a bat, could have been formed by the modification of someother animal with widely different habits and structure? Can we believethat natural selection could produce, on the one hand, an organ of triflingimportance, such as the tail of a giraffe, which serves as a fly-flapper,and, on the other hand, an organ so wonderful as the eye?

Thirdly, can instincts be acquired and modified through natural selection? What shall we say to the instinct which leads the bee to make cells, andwhich has practically anticipated the discoveries of profoundmathematicians?

Fourthly, how can we account for species, when crossed, being sterile andproducing sterile offspring, whereas, when varieties are crossed, theirfertility is unimpaired?

The two first heads will be here discussed; some miscellaneous objectionsin the following chapter; Instinct and Hybridism in the two succeedingchapters.

ON THE ABSENCE OR RARITY OF TRANSITIONAL VARIETIES.

As natural selection acts solely by the preservation of profitablemodifications, each new form will tend in a fully-stocked country to takethe place of, and finally to exterminate, its own less improved parent-formand other less-favoured forms with which it comes into competition. Thusextinction and natural selection go hand in hand. Hence, if we look ateach species as descended from some unknown form, both the parent and allthe transitional varieties will generally have been exterminated by thevery process of the formation and perfection of the new form.

But, as by this theory innumerable transitional forms must have existed,why do we not find them embedded in countless numbers in the crust of theearth? It will be more convenient to discuss this question in the chapteron the imperfection of the geological record; and I will here only statethat I believe the answer mainly lies in the record being incomparably lessperfect than is generally supposed. The crust of the earth is a vastmuseum; but the natural collections have been imperfectly made, and only atlong intervals of time.

But it may be urged that when several closely allied species inhabit thesame territory, we surely ought to find at the present time manytransitional forms. Let us take a simple case: in travelling from northto south over a continent, we generally meet at successive intervals withclosely allied or representative species, evidently filling nearly the sameplace in the natural economy of the land. These representative speciesoften meet and interlock; and as the one becomes rarer and rarer, the otherbecomes more and more frequent, till the one replaces the other. But if wecompare these species where they intermingle, they are generally asabsolutely distinct from each other in every detail of structure as arespecimens taken from the metropolis inhabited by each. By my theory theseallied species are descended from a common parent; and during the processof modification, each has become adapted to the conditions of life of itsown region, and has supplanted and exterminated its original parent-formand all the transitional varieties between its past and present states. Hence we ought not to expect at the present time to meet with numeroustransitional varieties in each region, though they must have existed there,and may be embedded there in a fossil condition. But in the intermediateregion, having intermediate conditions of life, why do we not now findclosely-linking intermediate varieties? This difficulty for a long timequite confounded me. But I think it can be in large part explained.

In the first place we should be extremely cautious in inferring, because anarea is now continuous, that it has been continuous during a long period. Geology would lead us to believe that most continents have been broken upinto islands even during the later tertiary periods; and in such islandsdistinct species might have been separately formed without the possibilityof intermediate varieties existing in the intermediate zones. By changesin the form of the land and of climate, marine areas now continuous mustoften have existed within recent times in a far less continuous and uniformcondition than at present. But I will pass over this way of escaping fromthe difficulty; for I believe that many perfectly defined species have beenformed on strictly continuous areas; though I do not doubt that theformerly broken condition of areas now continuous, has played an importantpart in the formation of new species, more especially with freely-crossingand wandering animals.

In looking at species as they are now distributed over a wide area, wegenerally find them tolerably numerous over a large territory, thenbecoming somewhat abruptly rarer and rarer on the confines, and finallydisappearing. Hence the neutral territory between two representativespecies is generally narrow in comparison with the territory proper toeach. We see the same fact in ascending mountains, and sometimes it isquite remarkable how abruptly, as Alph. De Candolle has observed, a commonalpine species disappears. The same fact has been noticed by E. Forbes insounding the depths of the sea with the dredge. To those who look atclimate and the physical conditions of life as the all-important elementsof distribution, these facts ought to cause surprise, as climate and heightor depth graduate away insensibly. But when we bear in mind that almostevery species, even in its metropolis, would increase immensely in numbers,were it not for other competing species; that nearly all either prey on orserve as prey for others; in short, that each organic being is eitherdirectly or indirectly related in the most important manner to otherorganic beings--we see that the range of the inhabitants of any country byno means exclusively depends on insensibly changing physical conditions,but in large part on the presence of other species, on which it lives, orby which it is destroyed, or with which it comes into competition; and asthese species are already defined objects, not blending one into another byinsensible gradations, the range of any one species, depending as it doeson the range of others, will tend to be sharply defined. Moreover, eachspecies on the confines of its range, where it exists in lessened numbers,will, during fluctuations in the number of its enemies or of its prey, orin the nature of the seasons, be extremely liable to utter extermination;and thus its geographical range will come to be still more sharply defined.

As allied or representative species, when inhabiting a continuous area, aregenerally distributed in such a manner that each has a wide range, with acomparatively narrow neutral territory between them, in which they becomerather suddenly rarer and rarer; then, as varieties do not essentiallydiffer from species, the same rule will probably apply to both; and if wetake a varying species inhabiting a very large area, we shall have to adapttwo varieties to two large areas, and a third variety to a narrowintermediate zone. The intermediate variety, consequently, will exist inlesser numbers from inhabiting a narrow and lesser area; and practically,as far as I can make out, this rule holds good with varieties in a state ofnature. I have met with striking instances of the rule in the case ofvarieties intermediate between well-marked varieties in the genus Balanus. And it would appear from information given me by Mr. Watson, Dr. Asa Gray,and Mr. Wollaston, that generally, when varieties intermediate between twoother forms occur, they are much rarer numerically than the forms whichthey connect. Now, if we may trust these facts and inferences, andconclude that varieties linking two other varieties together generally haveexisted in lesser numbers than the forms which they connect, then we canunderstand why intermediate varieties should not endure for very longperiods: why, as a general rule, they should be exterminated anddisappear, sooner than the forms which they originally linked together.

For any form existing in lesser numbers would, as already remarked, run agreater chance of being exterminated than one existing in large numbers;and in this particular case the intermediate form would be eminently liableto the inroads of closely allied forms existing on both sides of it. Butit is a far more important consideration, that during the process offurther modification, by which two varieties are supposed to be convertedand perfected into two distinct species, the two which exist in largernumbers, from inhabiting larger areas, will have a great advantage over theintermediate variety, which exists in smaller numbers in a narrow andintermediate zone. For forms existing in larger numbers will have a betterchance, within any given period, of presenting further favourablevariations for natural selection to seize on, than will the rarer formswhich exist in lesser numbers. Hence, the more common forms, in the racefor life, will tend to beat and supplant the less common forms, for thesewill be more slowly modified and improved. It is the same principle which,as I believe, accounts for the common species in each country, as shown inthe second chapter, presenting on an average a greater number ofwell-marked varieties than do the rarer species. I may illustrate what Imean by supposing three varieties of sheep to be kept, one adapted to anextensive mountainous region; a second to a comparatively narrow, hillytract; and a third to the wide plains at the base; and that the inhabitantsare all trying with equal steadiness and skill to improve their stocks byselection; the chances in this case will be strongly in favour of the greatholders on the mountains or on the plains improving their breeds morequickly than the small holders on the intermediate narrow, hilly tract; andconsequently the improved mountain or plain breed will soon take the placeof the less improved hill breed; and thus the two breeds, which originallyexisted in greater numbers, will come into close contact with each other,without the interposition of the supplanted, intermediate hill variety.

To sum up, I believe that species come to be tolerably well-definedobjects, and do not at any one period present an inextricable chaos ofvarying and intermediate links: first, because new varieties are veryslowly formed, for variation is a slow process, and natural selection cando nothing until favourable individual differences or variations occur, anduntil a place in the natural polity of the country can be better filled bysome modification of some one or more of its inhabitants. And such newplaces will depend on slow changes of climate, or on the occasionalimmigration of new inhabitants, and, probably, in a still more importantdegree, on some of the old inhabitants becoming slowly modified, with thenew forms thus produced and the old ones acting and reacting on each other. So that, in any one region and at any one time, we ought to see only a fewspecies presenting slight modifications of structure in some degreepermanent; and this assuredly we do see.

Secondly, areas now continuous must often have existed within the recentperiod as isolated portions, in which many forms, more especially among theclasses which unite for each birth and wander much, may have separatelybeen rendered sufficiently distinct to rank as representative species. Inthis case, intermediate varieties between the several representativespecies and their common parent, must formerly have existed within eachisolated portion of the land, but these links during the process of naturalselection will have been supplanted and exterminated, so that they will nolonger be found in a living state.

Thirdly, when two or more varieties have been formed in different portionsof a strictly continuous area, intermediate varieties will, it is probable,at first have been formed in the intermediate zones, but they willgenerally have had a short duration. For these intermediate varietieswill, from reasons already assigned (namely from what we know of the actualdistribution of closely allied or representative species, and likewise ofacknowledged varieties), exist in the intermediate zones in lesser numbersthan the varieties which they tend to connect. From this cause alone theintermediate varieties will be liable to accidental extermination; andduring the process of further modification through natural selection, theywill almost certainly be beaten and supplanted by the forms which theyconnect; for these, from existing in greater numbers will, in theaggregate, present more varieties, and thus be further improved throughnatural selection and gain further advantages.

Lastly, looking not to any one time, but at all time, if my theory be true,numberless intermediate varieties, linking closely together all the speciesof the same group, must assuredly have existed; but the very process ofnatural selection constantly tends, as has been so often remarked, toexterminate the parent forms and the intermediate links. Consequentlyevidence of their former existence could be found among fossil remains,which are preserved, as we shall attempt to show in a future chapter, in anextremely imperfect and intermittent record.

ON THE ORIGIN AND TRANSITION OF ORGANIC BEINGS WITH PECULIAR HABITS ANDSTRUCTURE.

It has been asked by the opponents of such views as I hold, how, forinstance, could a land carnivorous animal have been converted into one withaquatic habits; for how could the animal in its transitional state havesubsisted? It would be easy to show that there now exist carnivorousanimals presenting close intermediate grades from strictly terrestrial toaquatic habits; and as each exists by a struggle for life, it is clear thateach must be well adapted to its place in nature. Look at the Mustelavison of North America, which has webbed feet, and which resembles an otterin its fur, short legs, and form of tail; during summer this animal divesfor and preys on fish, but during the long winter it leaves the frozenwaters, and preys, like other polecats on mice and land animals. If adifferent case had been taken, and it had been asked how an insectivorousquadruped could possibly have been converted into a flying bat, thequestion would have been far more difficult to answer. Yet I think suchdifficulties have little weight.

Here, as on other occasions, I lie under a heavy disadvantage, for, out ofthe many striking cases which I have collected, I can give only one or twoinstances of transitional habits and structures in allied species; and ofdiversified habits, either constant or occasional, in the same species. And it seems to me that nothing less than a long list of such cases issufficient to lessen the difficulty in any particular case like that of thebat.

Look at the family of squirrels; here we have the finest gradation fromanimals with their tails only slightly flattened, and from others, as SirJ. Richardson has remarked, with the posterior part of their bodies ratherwide and with the skin on their flanks rather full, to the so-called flyingsquirrels; and flying squirrels have their limbs and even the base of thetail united by a broad expanse of skin, which serves as a parachute andallows them to glide through the air to an astonishing distance from treeto tree. We cannot doubt that each structure is of use to each kind ofsquirrel in its own country, by enabling it to escape birds or beasts ofprey, or to collect food more quickly, or, as there is reason to believe,to lessen the danger from occasional falls. But it does not follow fromthis fact that the structure of each squirrel is the best that it ispossible to conceive under all possible conditions. Let the climate andvegetation change, let other competing rodents or new beasts of preyimmigrate, or old ones become modified, and all analogy would lead us tobelieve that some, at least, of the squirrels would decrease in numbers orbecome exterminated, unless they also become modified and improved instructure in a corresponding manner. Therefore, I can see no difficulty,more especially under changing conditions of life, in the continuedpreservation of individuals with fuller and fuller flank-membranes, eachmodification being useful, each being propagated, until, by the accumulatedeffects of this process of natural selection, a perfect so-called flyingsquirrel was produced.

Now look at the Galeopithecus or so-called flying lemur, which was formerlyranked among bats, but is now believed to belong to the Insectivora. Anextremely wide flank-membrane stretches from the corners of the jaw to thetail, and includes the limbs with the elongated fingers. This flank-membrane is furnished with an extensor muscle. Although no graduated linksof structure, fitted for gliding through the air, now connect theGaleopithecus with the other Insectivora, yet there is no difficulty insupposing that such links formerly existed, and that each was developed inthe same manner as with the less perfectly gliding squirrels; each grade ofstructure having been useful to its possessor. Nor can I see anyinsuperable difficulty in further believing it possible that themembrane-connected fingers and fore-arm of the Galeopithecus might havebeen greatly lengthened by natural selection; and this, as far as theorgans of flight are concerned, would have converted the animal into a bat. In certain bats in which the wing-membrane extends from the top of theshoulder to the tail and includes the hind-legs, we perhaps see traces ofan apparatus originally fitted for gliding through the air rather than forflight.

If about a dozen genera of birds were to become extinct, who would haveventured to surmise that birds might have existed which used their wingssolely as flappers, like the logger headed duck (Micropterus of Eyton); asfins in the water and as front legs on the land, like the penguin; assails, like the ostrich; and functionally for no purpose, like the apteryx? Yet the structure of each of these birds is good for it, under theconditions of life to which it is exposed, for each has to live by astruggle: but it is not necessarily the best possible under all possibleconditions. It must not be inferred from these remarks that any of thegrades of wing-structure here alluded to, which perhaps may all be theresult of disuse, indicate the steps by which birds actually acquired theirperfect power of flight; but they serve to show what diversified means oftransition are at least possible.

Seeing that a few members of such water-breathing classes as the Crustaceaand Mollusca are adapted to live on the land; and seeing that we haveflying birds and mammals, flying insects of the most diversified types, andformerly had flying reptiles, it is conceivable that flying-fish, which nowglide far through the air, slightly rising and turning by the aid of theirfluttering fins, might have been modified into perfectly winged animals. If this had been effected, who would have ever imagined that in an earlytransitional state they had been inhabitants of the open ocean, and hadused their incipient organs of flight exclusively, so far as we know, toescape being devoured by other fish?

When we see any structure highly perfected for any particular habit, as thewings of a bird for flight, we should bear in mind that animals displayingearly transitional grades of the structure will seldom have survived to thepresent day, for they will have been supplanted by their successors, whichwere gradually rendered more perfect through natural selection. Furthermore, we may conclude that transitional states between structuresfitted for very different habits of life will rarely have been developed atan early period in great numbers and under many subordinate forms. Thus,to return to our imaginary illustration of the flying-fish, it does notseem probable that fishes capable of true flight would have been developedunder many subordinate forms, for taking prey of many kinds in many ways,on the land and in the water, until their organs of flight had come to ahigh stage of perfection, so as to have given them a decided advantage overother animals in the battle for life. Hence the chance of discoveringspecies with transitional grades of structure in a fossil condition willalways be less, from their having existed in lesser numbers, than in thecase of species with fully developed structures.

I will now give two or three instances, both of diversified and of changedhabits, in the individuals of the same species. In either case it would beeasy for natural selection to adapt the structure of the animal to itschanged habits, or exclusively to one of its several habits. It is,however, difficult to decide and immaterial for us, whether habitsgenerally change first and structure afterwards; or whether slightmodifications of structure lead to changed habits; both probably oftenoccurring almost simultaneously. Of cases of changed habits it willsuffice merely to allude to that of the many British insects which now feedon exotic plants, or exclusively on artificial substances. Of diversifiedhabits innumerable instances could be given: I have often watched a tyrantflycatcher (Saurophagus sulphuratus) in South America, hovering over onespot and then proceeding to another, like a kestrel, and at other timesstanding stationary on the margin of water, and then dashing into it like akingfisher at a fish. In our own country the larger titmouse (Parus major)may be seen climbing branches, almost like a creeper; it sometimes, like ashrike, kills small birds by blows on the head; and I have many times seenand heard it hammering the seeds of the yew on a branch, and thus breakingthem like a nuthatch. In North America the black bear was seen by Hearneswimming for hours with widely open mouth, thus catching, almost like awhale, insects in the water.

As we sometimes see individuals following habits different from thoseproper to their species and to the other species of the same genus, wemight expect that such individuals would occasionally give rise to newspecies, having anomalous habits, and with their structure either slightlyor considerably modified from that of their type. And such instances occurin nature. Can a more striking instance of adaptation be given than thatof a woodpecker for climbing trees and seizing insects in the chinks of thebark? Yet in North America there are woodpeckers which feed largely onfruit, and others with elongated wings which chase insects on the wing. Onthe plains of La Plata, where hardly a tree grows, there is a woodpecker(Colaptes campestris) which has two toes before and two behind, a long-pointed tongue, pointed tail-feathers, sufficiently stiff to support thebird in a vertical position on a post, but not so stiff as in the typicalwood-peckers, and a straight, strong beak. The beak, however, is not sostraight or so strong as in the typical woodpeckers but it is strong enoughto bore into wood. Hence this Colaptes, in all the essential parts of itsstructure, is a woodpecker. Even in such trifling characters as thecolouring, the harsh tone of the voice, and undulatory flight, its closeblood-relationship to our common woodpecker is plainly declared; yet, as Ican assert, not only from my own observations, but from those of theaccurate Azara, in certain large districts it does not climb trees, and itmakes its nest in holes in banks! In certain other districts, however,this same woodpecker, as Mr. Hudson states, frequents trees, and boresholes in the trunk for its nest. I may mention as another illustration ofthe varied habits of this genus, that a Mexican Colaptes has been describedby De Saussure as boring holes into hard wood in order to lay up a store ofacorns.

Petrels are the most aerial and oceanic of birds, but, in the quiet soundsof Tierra del Fuego, the Puffinuria berardi, in its general habits, in itsastonishing power of diving, in its manner of swimming and of flying whenmade to take flight, would be mistaken by any one for an auk or a grebe;nevertheless, it is essentially a petrel, but with many parts of itsorganisation profoundly modified in relation to its new habits of life;whereas the woodpecker of La Plata has had its structure only slightlymodified. In the case of the water-ouzel, the acutest observer, byexamining its dead body, would never have suspected its sub-aquatic habits;yet this bird, which is allied to the thrush family, subsists bydiving,--using its wings under water and grasping stones with its feet. All the members of the great order of Hymenopterous insects areterrestrial, excepting the genus Proctotrupes, which Sir John Lubbock hasdiscovered to be aquatic in its habits; it often enters the water and divesabout by the use not of its legs but of its wings, and remains as long asfour hours beneath the surface; yet it exhibits no modification instructure in accordance with its abnormal habits.

He who believes that each being has been created as we now see it, mustoccasionally have felt surprise when he has met with an animal havinghabits and structure not in agreement. What can be plainer than that thewebbed feet of ducks and geese are formed for swimming? Yet there areupland geese with webbed feet which rarely go near the water; and no oneexcept Audubon, has seen the frigate-bird, which has all its four toeswebbed, alight on the surface of the ocean. On the other hand, grebes andcoots are eminently aquatic, although their toes are only bordered bymembrane. What seems plainer than that the long toes, not furnished withmembrane, of the Grallatores, are formed for walking over swamps andfloating plants. The water-hen and landrail are members of this order, yetthe first is nearly as aquatic as the coot, and the second is nearly asterrestrial as the quail or partridge. In such cases, and many otherscould be given, habits have changed without a corresponding change ofstructure. The webbed feet of the upland goose may be said to have becomealmost rudimentary in function, though not in structure. In thefrigate-bird, the deeply scooped membrane between the toes shows thatstructure has begun to change.

He who believes in separate and innumerable acts of creation may say, thatin these cases it has pleased the Creator to cause a being of one type totake the place of one belonging to another type; but this seems to me onlyrestating the fact in dignified language. He who believes in the strugglefor existence and in the principle of natural selection, will acknowledgethat every organic being is constantly endeavouring to increase in numbers;and that if any one being varies ever so little, either in habits orstructure, and thus gains an advantage over some other inhabitant of thesame country, it will seize on the place of that inhabitant, howeverdifferent that may be from its own place. Hence it will cause him nosurprise that there should be geese and frigate-birds with webbed feet,living on the dry land and rarely alighting on the water, that there shouldbe long-toed corncrakes, living in meadows instead of in swamps; that thereshould be woodpeckers where hardly a tree grows; that there should bediving thrushes and diving Hymenoptera, and petrels with the habits ofauks.

ORGANS OF EXTREME PERFECTION AND COMPLICATION.

To suppose that the eye with all its inimitable contrivances for adjustingthe focus to different distances, for admitting different amounts of light,and for the correction of spherical and chromatic aberration, could havebeen formed by natural selection, seems, I freely confess, absurd in thehighest degree. When it was first said that the sun stood still and theworld turned round, the common sense of mankind declared the doctrinefalse; but the old saying of Vox populi, vox Dei, as every philosopherknows, cannot be trusted in science. Reason tells me, that if numerousgradations from a simple and imperfect eye to one complex and perfect canbe shown to exist, each grade being useful to its possessor, as iscertainly the case; if further, the eye ever varies and the variations beinherited, as is likewise certainly the case; and if such variations shouldbe useful to any animal under changing conditions of life, then thedifficulty of believing that a perfect and complex eye could be formed bynatural selection, though insuperable by our imagination, should not beconsidered as subversive of the theory. How a nerve comes to be sensitiveto light, hardly concerns us more than how life itself originated; but Imay remark that, as some of the lowest organisms in which nerves cannot bedetected, are capable of perceiving light, it does not seem impossible thatcertain sensitive elements in their sarcode should become aggregated anddeveloped into nerves, endowed with this special sensibility.

In searching for the gradations through which an organ in any species hasbeen perfected, we ought to look exclusively to its lineal progenitors; butthis is scarcely ever possible, and we are forced to look to other speciesand genera of the same group, that is to the collateral descendants fromthe same parent-form, in order to see what gradations are possible, and forthe chance of some gradations having been transmitted in an unaltered orlittle altered condition. But the state of the same organ in distinctclasses may incidentally throw light on the steps by which it has beenperfected.

The simplest organ which can be called an eye consists of an optic nerve,surrounded by pigment-cells and covered by translucent skin, but withoutany lens or other refractive body. We may, however, according to M.Jourdain, descend even a step lower and find aggregates of pigment-cells,apparently serving as organs of vision, without any nerves, and restingmerely on sarcodic tissue. Eyes of the above simple nature are not capableof distinct vision, and serve only to distinguish light from darkness. Incertain star-fishes, small depressions in the layer of pigment whichsurrounds the nerve are filled, as described by the author just quoted,with transparent gelatinous matter, projecting with a convex surface, likethe cornea in the higher animals. He suggests that this serves not to forman image, but only to concentrate the luminous rays and render theirperception more easy. In this concentration of the rays we gain the firstand by far the most important step towards the formation of a true,picture-forming eye; for we have only to place the naked extremity of theoptic nerve, which in some of the lower animals lies deeply buried in thebody, and in some near the surface, at the right distance from theconcentrating apparatus, and an image will be formed on it.

In the great class of the Articulata, we may start from an optic nervesimply coated with pigment, the latter sometimes forming a sort of pupil,but destitute of lens or other optical contrivance. With insects it is nowknown that the numerous facets on the cornea of their great compound eyesform true lenses, and that the cones include curiously modified nervousfilaments. But these organs in the Articulata are so much diversified thatMuller formerly made three main classes with seven subdivisions, besides afourth main class of aggregated simple eyes.

When we reflect on these facts, here given much too briefly, with respectto the wide, diversified, and graduated range of structure in the eyes ofthe lower animals; and when we bear in mind how small the number of allliving forms must be in comparison with those which have become extinct,the difficulty ceases to be very great in believing that natural selectionmay have converted the simple apparatus of an optic nerve, coated withpigment and invested by transparent membrane, into an optical instrument asperfect as is possessed by any member of the Articulata class.

He who will go thus far, ought not to hesitate to go one step further, ifhe finds on finishing this volume that large bodies of facts, otherwiseinexplicable, can be explained by the theory of modification throughnatural selection; he ought to admit that a structure even as perfect as aneagle's eye might thus be formed, although in this case he does not knowthe transitional states. It has been objected that in order to modify theeye and still preserve it as a perfect instrument, many changes would haveto be effected simultaneously, which, it is assumed, could not be donethrough natural selection; but as I have attempted to show in my work onthe variation of domestic animals, it is not necessary to suppose that themodifications were all simultaneous, if they were extremely slight andgradual. Different kinds of modification would, also, serve for the samegeneral purpose: as Mr. Wallace has remarked, "If a lens has too short ortoo long a focus, it may be amended either by an alteration of curvature,or an alteration of density; if the curvature be irregular, and the rays donot converge to a point, then any increased regularity of curvature will bean improvement. So the contraction of the iris and the muscular movementsof the eye are neither of them essential to vision, but only improvementswhich might have been added and perfected at any stage of the constructionof the instrument." Within the highest division of the animal kingdom,namely, the Vertebrata, we can start from an eye so simple, that itconsists, as in the lancelet, of a little sack of transparent skin,furnished with a nerve and lined with pigment, but destitute of any otherapparatus. In fishes and reptiles, as Owen has remarked, "The range ofgradation of dioptric structures is very great." It is a significant factthat even in man, according to the high authority of Virchow, the beautifulcrystalline lens is formed in the embryo by an accumulation of epidermiccells, lying in a sack-like fold of the skin; and the vitreous body isformed from embryonic subcutaneous tissue. To arrive, however, at a justconclusion regarding the formation of the eye, with all its marvellous yetnot absolutely perfect characters, it is indispensable that the reasonshould conquer the imagination; but I have felt the difficulty far tokeenly to be surprised at others hesitating to extend the principle ofnatural selection to so startling a length.

It is scarcely possible to avoid comparing the eye with a telescope. Weknow that this instrument has been perfected by the long-continued effortsof the highest human intellects; and we naturally infer that the eye hasbeen formed by a somewhat analogous process. But may not this inference bepresumptuous? Have we any right to assume that the Creator works byintellectual powers like those of man? If we must compare the eye to anoptical instrument, we ought in imagination to take a thick layer oftransparent tissue, with spaces filled with fluid, and with a nervesensitive to light beneath, and then suppose every part of this layer to becontinually changing slowly in density, so as to separate into layers ofdifferent densities and thicknesses, placed at different distances fromeach other, and with the surfaces of each layer slowly changing in form. Further we must suppose that there is a power, represented by naturalselection or the survival of the fittest, always intently watching eachslight alteration in the transparent layers; and carefully preserving eachwhich, under varied circumstances, in any way or degree, tends to produce adistincter image. We must suppose each new state of the instrument to bemultiplied by the million; each to be preserved until a better is produced,and then the old ones to be all destroyed. In living bodies, variationwill cause the slight alteration, generation will multiply them almostinfinitely, and natural selection will pick out with unerring skill eachimprovement. Let this process go on for millions of years; and during eachyear on millions of individuals of many kinds; and may we not believe thata living optical instrument might thus be formed as superior to one ofglass, as the works of the Creator are to those of man?

MODES Of TRANSITION.

If it could be demonstrated that any complex organ existed, which could notpossibly have been formed by numerous, successive, slight modifications, mytheory would absolutely break down. But I can find out no such case. Nodoubt many organs exist of which we do not know the transitional grades,more especially if we look to much-isolated species, around which,according to the theory, there has been much extinction. Or again, if wetake an organ common to all the members of a class, for in this latter casethe organ must have been originally formed at a remote period, since whichall the many members of the class have been developed; and in order todiscover the early transitional grades through which the organ has passed,we should have to look to very ancient ancestral forms, long since becomeextinct.

We should be extremely cautious in concluding that an organ could not havebeen formed by transitional gradations of some kind. Numerous cases couldbe given among the lower animals of the same organ performing at the sametime wholly distinct functions; thus in the larva of the dragon-fly and inthe fish Cobites the alimentary canal respires, digests, and excretes. Inthe Hydra, the animal may be turned inside out, and the exterior surfacewill then digest and the stomach respire. In such cases natural selectionmight specialise, if any advantage were thus gained, the whole or part ofan organ, which had previously performed two functions, for one functionalone, and thus by insensible steps greatly change its nature. Many plantsare known which regularly produce at the same time differently constructedflowers; and if such plants were to produce one kind alone, a great changewould be effected with comparative suddenness in the character of thespecies. It is, however, probable that the two sorts of flowers borne bythe same plant were originally differentiated by finely graduated steps,which may still be followed in some few cases.

Again, two distinct organs, or the same organ under two very differentforms, may simultaneously perform in the same individual the same function,and this is an extremely important means of transition: to give oneinstance--there are fish with gills or branchiae that breathe the airdissolved in the water, at the same time that they breathe free air intheir swim-bladders, this latter organ being divided by highly vascularpartitions and having a ductus pneumaticus for the supply of air. To giveanother instance from the vegetable kingdom: plants climb by threedistinct means, by spirally twining, by clasping a support with theirsensitive tendrils, and by the emission of aerial rootlets; these threemeans are usually found in distinct groups, but some few species exhibittwo of the means, or even all three, combined in the same individual. Inall such cases one of the two organs might readily be modified andperfected so as to perform all the work, being aided during the progress ofmodification by the other organ; and then this other organ might bemodified for some other and quite distinct purpose, or be whollyobliterated.

The illustration of the swim-bladder in fishes is a good one, because itshows us clearly the highly important fact that an organ originallyconstructed for one purpose, namely flotation, may be converted into onefor a widely different purpose, namely respiration. The swim-bladder has,also, been worked in as an accessory to the auditory organs of certainfishes. All physiologists admit that the swim-bladder is homologous, or"ideally similar" in position and structure with the lungs of the highervertebrate animals: hence there is no reason to doubt that the swim-bladder has actually been converted into lungs, or an organ usedexclusively for respiration.

According to this view it may be inferred that all vertebrate animals withtrue lungs are descended by ordinary generation from an ancient and unknownprototype which was furnished with a floating apparatus or swim-bladder. We can thus, as I infer from Professor Owen's interesting description ofthese parts, understand the strange fact that every particle of food anddrink which we swallow has to pass over the orifice of the trachea, withsome risk of falling into the lungs, notwithstanding the beautifulcontrivance by which the glottis is closed. In the higher Vertebrata thebranchiae have wholly disappeared--but in the embryo the slits on the sidesof the neck and the loop-like course of the arteries still mark theirformer position. But it is conceivable that the now utterly lost branchiaemight have been gradually worked in by natural selection for some distinctpurpose: for instance, Landois has shown that the wings of insects aredeveloped from the trachea; it is therefore highly probable that in thisgreat class organs which once served for respiration have been actuallyconverted into organs for flight.

In considering transitions of organs, it is so important to bear in mindthe probability of conversion from one function to another, that I willgive another instance. Pedunculated cirripedes have two minute folds ofskin, called by me the ovigerous frena, which serve, through the means of asticky secretion, to retain the eggs until they are hatched within thesack. These cirripedes have no branchiae, the whole surface of the bodyand of the sack, together with the small frena, serving for respiration. The Balanidae or sessile cirripedes, on the other hand, have no ovigerousfrena, the eggs lying loose at the bottom of the sack, within thewell-enclosed shell; but they have, in the same relative position with thefrena, large, much-folded membranes, which freely communicate with thecirculatory lacunae of the sack and body, and which have been considered byall naturalists to act as branchiae. Now I think no one will dispute thatthe ovigerous frena in the one family are strictly homologous with thebranchiae of the other family; indeed, they graduate into each other. Therefore it need not be doubted that the two little folds of skin, whichoriginally served as ovigerous frena, but which, likewise, very slightlyaided in the act of respiration, have been gradually converted by naturalselection into branchiae, simply through an increase in their size and theobliteration of their adhesive glands. If all pedunculated cirripedes hadbecome extinct, and they have suffered far more extinction than havesessile cirripedes, who would ever have imagined that the branchiae in thislatter family had originally existed as organs for preventing the ova frombeing washed out of the sack?

There is another possible mode of transition, namely, through theacceleration or retardation of the period of reproduction. This has latelybeen insisted on by Professor Cope and others in the United States. It isnow known that some animals are capable of reproduction at a very earlyage, before they have acquired their perfect characters; and if this powerbecame thoroughly well developed in a species, it seems probable that theadult stage of development would sooner or later be lost; and in this case,especially if the larva differed much from the mature form, the characterof the species would be greatly changed and degraded. Again, not a fewanimals, after arriving at maturity, go on changing in character duringnearly their whole lives. With mammals, for instance, the form of theskull is often much altered with age, of which Dr. Murie has given somestriking instances with seals. Every one knows how the horns of stagsbecome more and more branched, and the plumes of some birds become morefinely developed, as they grow older. Professor Cope states that the teethof certain lizards change much in shape with advancing years. Withcrustaceans not only many trivial, but some important parts assume a newcharacter, as recorded by Fritz Muller, after maturity. In all such cases--and many could be given--if the age for reproduction were retarded, thecharacter of the species, at least in its adult state, would be modified;nor is it improbable that the previous and earlier stages of developmentwould in some cases be hurried through and finally lost. Whether specieshave often or ever been modified through this comparatively sudden mode oftransition, I can form no opinion; but if this has occurred, it is probablethat the differences between the young and the mature, and between themature and the old, were primordially acquired by graduated steps.

SPECIAL DIFFICULTIES OF THE THEORY OF NATURAL SELECTION.

Although we must be extremely cautious in concluding that any organ couldnot have been produced by successive, small, transitional gradations, yetundoubtedly serious cases of difficulty occur.

One of the most serious is that of neuter insects, which are oftendifferently constructed from either the males or fertile females; but thiscase will be treated of in the next chapter. The electric organs of fishesoffer another case of special difficulty; for it is impossible to conceiveby what steps these wondrous organs have been produced. But this is notsurprising, for we do not even know of what use they are. In the gymnotusand torpedo they no doubt serve as powerful means of defence, and perhapsfor securing prey; yet in the ray, as observed by Matteucci, an analogousorgan in the tail manifests but little electricity, even when the animal isgreatly irritated; so little that it can hardly be of any use for the abovepurposes. Moreover, in the ray, besides the organ just referred to, thereis, as Dr. R. McDonnell has shown, another organ near the head, not knownto be electrical, but which appears to be the real homologue of theelectric battery in the torpedo. It is generally admitted that thereexists between these organs and ordinary muscle a close analogy, inintimate structure, in the distribution of the nerves, and in the manner inwhich they are acted on by various reagents. It should, also, beespecially observed that muscular contraction is accompanied by anelectrical discharge; and, as Dr. Radcliffe insists, "in the electricalapparatus of the torpedo during rest, there would seem to be a charge inevery respect like that which is met with in muscle and nerve during therest, and the discharge of the torpedo, instead of being peculiar, may beonly another form of the discharge which attends upon the action of muscleand motor nerve." Beyond this we cannot at present go in the way ofexplanation; but as we know so little about the uses of these organs, andas we know nothing about the habits and structure of the progenitors of theexisting electric fishes, it would be extremely bold to maintain that noserviceable transitions are possible by which these organs might have beengradually developed.

These organs appear at first to offer another and far more seriousdifficulty; for they occur in about a dozen kinds of fish, of which severalare widely remote in their affinities. When the same organ is found inseveral members of the same class, especially if in members having verydifferent habits of life, we may generally attribute its presence toinheritance from a common ancestor; and its absence in some of the membersto loss through disuse or natural selection. So that, if the electricorgans had been inherited from some one ancient progenitor, we might haveexpected that all electric fishes would have been specially related to eachother; but this is far from the case. Nor does geology at all lead to thebelief that most fishes formerly possessed electric organs, which theirmodified descendants have now lost. But when we look at the subject moreclosely, we find in the several fishes provided with electric organs, thatthese are situated in different parts of the body, that they differ inconstruction, as in the arrangement of the plates, and, according toPacini, in the process or means by which the electricity is excited--andlastly, in being supplied with nerves proceeding from different sources,and this is perhaps the most important of all the differences. Hence inthe several fishes furnished with electric organs, these cannot beconsidered as homologous, but only as analogous in function. Consequentlythere is no reason to suppose that they have been inherited from a commonprogenitor; for had this been the case they would have closely resembledeach other in all respects. Thus the difficulty of an organ, apparentlythe same, arising in several remotely allied species, disappears, leavingonly the lesser yet still great difficulty: namely, by what graduatedsteps these organs have been developed in each separate group of fishes.

The luminous organs which occur in a few insects, belonging to widelydifferent families, and which are situated in different parts of the body,offer, under our present state of ignorance, a difficulty almost exactlyparallel with that of the electric organs. Other similar cases could begiven; for instance in plants, the very curious contrivance of a mass ofpollen-grains, borne on a foot-stalk with an adhesive gland, is apparentlythe same in Orchis and Asclepias, genera almost as remote as is possibleamong flowering plants; but here again the parts are not homologous. Inall cases of beings, far removed from each other in the scale oforganisation, which are furnished with similar and peculiar organs, it willbe found that although the general appearance and function of the organsmay be the same, yet fundamental differences between them can always bedetected. For instance, the eyes of Cephalopods or cuttle-fish and ofvertebrate animals appear wonderfully alike; and in such widely sunderedgroups no part of this resemblance can be due to inheritance from a commonprogenitor. Mr. Mivart has advanced this case as one of specialdifficulty, but I am unable to see the force of his argument. An organ forvision must be formed of transparent tissue, and must include some sort oflens for throwing an image at the back of a darkened chamber. Beyond thissuperficial resemblance, there is hardly any real similarity between theeyes of cuttle-fish and vertebrates, as may be seen by consulting Hensen'sadmirable memoir on these organs in the Cephalopoda. It is impossible forme here to enter on details, but I may specify a few of the points ofdifference. The crystalline lens in the higher cuttle-fish consists of twoparts, placed one behind the other like two lenses, both having a verydifferent structure and disposition to what occurs in the vertebrata. Theretina is wholly different, with an actual inversion of the elementalparts, and with a large nervous ganglion included within the membranes ofthe eye. The relations of the muscles are as different as it is possibleto conceive, and so in other points. Hence it is not a little difficult todecide how far even the same terms ought to be employed in describing theeyes of the Cephalopoda and Vertebrata. It is, of course, open to any oneto deny that the eye in either case could have been developed through thenatural selection of successive slight variations; but if this be admittedin the one case it is clearly possible in the other; and fundamentaldifferences of structure in the visual organs of two groups might have beenanticipated, in accordance with this view of their manner of formation. Astwo men have sometimes independently hit on the same invention, so in theseveral foregoing cases it appears that natural selection, working for thegood of each being, and taking advantage of all favourable variations, hasproduced similar organs, as far as function is concerned, in distinctorganic beings, which owe none of their structure in common to inheritancefrom a common progenitor.

Fritz Muller, in order to test the conclusions arrived at in this volume,has followed out with much care a nearly similar line of argument. Severalfamilies of crustaceans include a few species, possessing an air-breathingapparatus and fitted to live out of the water. In two of these families,which were more especially examined by Muller, and which are nearly relatedto each other, the species agree most closely in all important characters: namely in their sense organs, circulating systems, in the position of thetufts of hair within their complex stomachs, and lastly in the wholestructure of the water-breathing branchiae, even to the microscopical hooksby which they are cleansed. Hence it might have been expected that in thefew species belonging to both families which live on the land, the equallyimportant air-breathing apparatus would have been the same; for why shouldthis one apparatus, given for the same purpose, have been made to differ,while all the other important organs were closely similar, or rather,identical.

Fritz Muller argues that this close similarity in so many points ofstructure must, in accordance with the views advanced by me, be accountedfor by inheritance from a common progenitor. But as the vast majority ofthe species in the above two families, as well as most other crustaceans,are aquatic in their habits, it is improbable in the highest degree thattheir common progenitor should have been adapted for breathing air. Mullerwas thus led carefully to examine the apparatus in the air-breathingspecies; and he found it to differ in each in several important points, asin the position of the orifices, in the manner in which they are opened andclosed, and in some accessory details. Now such differences areintelligible, and might even have been expected, on the supposition thatspecies belonging to distinct families had slowly become adapted to livemore and more out of water, and to breathe the air. For these species,from belonging to distinct families, would have differed to a certainextent, and in accordance with the principle that the nature of eachvariation depends on two factors, viz., the nature of the organism and thatof the surrounding conditions, their variability assuredly would not havebeen exactly the same. Consequently natural selection would have haddifferent materials or variations to work on, in order to arrive at thesame functional result; and the structures thus acquired would almostnecessarily have differed. On the hypothesis of separate acts of creationthe whole case remains unintelligible. This line of argument seems to havehad great weight in leading Fritz Muller to accept the views maintained byme in this volume.

Another distinguished zoologist, the late Professor Claparede, has arguedin the same manner, and has arrived at the same result. He shows thatthere are parasitic mites (Acaridae), belonging to distinct sub-familiesand families, which are furnished with hair-claspers. These organs musthave been independently developed, as they could not have been inheritedfrom a common progenitor; and in the several groups they are formed by themodification of the fore legs, of the hind legs, of the maxillae or lips,and of appendages on the under side of the hind part of the body.

In the foregoing cases, we see the same end gained and the same functionperformed, in beings not at all or only remotely allied, by organs inappearance, though not in development, closely similar. On the other hand,it is a common rule throughout nature that the same end should be gained,even sometimes in the case of closely related beings, by the mostdiversified means. How differently constructed is the feathered wing of abird and the membrane-covered wing of a bat; and still more so the fourwings of a butterfly, the two wings of a fly, and the two wings with theelytra of a beetle. Bivalve shells are made to open and shut, but on whata number of patterns is the hinge constructed, from the long row of neatlyinterlocking teeth in a Nucula to the simple ligament of a Mussel! Seedsare disseminated by their minuteness, by their capsule being converted intoa light balloon-like envelope, by being embedded in pulp or flesh, formedof the most diverse parts, and rendered nutritious, as well asconspicuously coloured, so as to attract and be devoured by birds, byhaving hooks and grapnels of many kinds and serrated awns, so as to adhereto the fur of quadrupeds, and by being furnished with wings and plumes, asdifferent in shape as they are elegant in structure, so as to be wafted byevery breeze. I will give one other instance: for this subject of thesame end being gained by the most diversified means well deservesattention. Some authors maintain that organic beings have been formed inmany ways for the sake of mere variety, almost like toys in a shop, butsuch a view of nature is incredible. With plants having separated sexes,and with those in which, though hermaphrodites, the pollen does notspontaneously fall on the stigma, some aid is necessary for theirfertilisation. With several kinds this is effected by the pollen-grains,which are light and incoherent, being blown by the wind through mere chanceon to the stigma; and this is the simplest plan which can well beconceived. An almost equally simple, though very different plan occurs inmany plants in which a symmetrical flower secretes a few drops of nectar,and is consequently visited by insects; and these carry the pollen from theanthers to the stigma.

>From this simple stage we may pass through an inexhaustible number ofcontrivances, all for the same purpose and effected in essentially the samemanner, but entailing changes in every part of the flower. The nectar maybe stored in variously shaped receptacles, with the stamens and pistilsmodified in many ways, sometimes forming trap-like contrivances, andsometimes capable of neatly adapted movements through irritability orelasticity. From such structures we may advance till we come to such acase of extraordinary adaptation as that lately described by Dr. Cruger inthe Coryanthes. This orchid has part of its labellum or lower lip hollowedout into a great bucket, into which drops of almost pure water continuallyfall from two secreting horns which stand above it; and when the bucket ishalf-full, the water overflows by a spout on one side. The basal part ofthe labellum stands over the bucket, and is itself hollowed out into a sortof chamber with two lateral entrances; within this chamber there arecurious fleshy ridges. The most ingenious man, if he had not witnessedwhat takes place, could never have imagined what purpose all these partsserve. But Dr. Cruger saw crowds of large humble-bees visiting thegigantic flowers of this orchid, not in order to suck nectar, but to gnawoff the ridges within the chamber above the bucket; in doing this theyfrequently pushed each other into the bucket, and their wings being thuswetted they could not fly away, but were compelled to crawl out through thepassage formed by the spout or overflow. Dr. Cruger saw a "continualprocession" of bees thus crawling out of their involuntary bath. Thepassage is narrow, and is roofed over by the column, so that a bee, inforcing its way out, first rubs its back against the viscid stigma and thenagainst the viscid glands of the pollen-masses. The pollen-masses are thusglued to the back of the bee which first happens to crawl out through thepassage of a lately expanded flower, and are thus carried away. Dr. Crugersent me a flower in spirits of wine, with a bee which he had killed beforeit had quite crawled out, with a pollen-mass still fastened to its back. When the bee, thus provided, flies to another flower, or to the same flowera second time, and is pushed by its comrades into the bucket and thencrawls out by the passage, the pollen-mass necessarily comes first intocontact with the viscid stigma, and adheres to it, and the flower isfertilised. Now at last we see the full use of every part of the flower,of the water-secreting horns of the bucket half-full of water, whichprevents the bees from flying away, and forces them to crawl out throughthe spout, and rub against the properly placed viscid pollen-masses and theviscid stigma.

The construction of the flower in another closely allied orchid, namely,the Catasetum, is widely different, though serving the same end; and isequally curious. Bees visit these flowers, like those of the Coryanthes,in order to gnaw the labellum; in doing this they inevitably touch a long,tapering, sensitive projection, or, as I have called it, the antenna. Thisantenna, when touched, transmits a sensation or vibration to a certainmembrane which is instantly ruptured; this sets free a spring by which thepollen-mass is shot forth, like an arrow, in the right direction, andadheres by its viscid extremity to the back of the bee. The pollen-mass ofthe male plant (for the sexes are separate in this orchid) is thus carriedto the flower of the female plant, where it is brought into contact withthe stigma, which is viscid enough to break certain elastic threads, andretain the pollen, thus effecting fertilisation.

How, it may be asked, in the foregoing and in innumerable other instances,can we understand the graduated scale of complexity and the multifariousmeans for gaining the same end. The answer no doubt is, as alreadyremarked, that when two forms vary, which already differ from each other insome slight degree, the variability will not be of the same exact nature,and consequently the results obtained through natural selection for thesame general purpose will not be the same. We should also bear in mindthat every highly developed organism has passed through many changes; andthat each modified structure tends to be inherited, so that eachmodification will not readily be quite lost, but may be again and againfurther altered. Hence, the structure of each part of each species, forwhatever purpose it may serve, is the sum of many inherited changes,through which the species has passed during its successive adaptations tochanged habits and conditions of life.

Finally, then, although in many cases it is most difficult even toconjecture by what transitions organs could have arrived at their presentstate; yet, considering how small the proportion of living and known formsis to the extinct and unknown, I have been astonished how rarely an organcan be named, towards which no transitional grade is known to lead. It iscertainly true, that new organs appearing as if created for some specialpurpose rarely or never appear in any being; as indeed is shown by thatold, but somewhat exaggerated, canon in natural history of "Natura nonfacit saltum." We meet with this admission in the writings of almost everyexperienced naturalist; or, as Milne Edwards has well expressed it, "Natureis prodigal in variety, but niggard in innovation." Why, on the theory ofCreation, should there be so much variety and so little real novelty? Whyshould all the parts and organs of many independent beings, each supposedto have been separately created for its own proper place in nature, be socommonly linked together by graduated steps? Why should not Nature take asudden leap from structure to structure? On the theory of naturalselection, we can clearly understand why she should not; for naturalselection acts only by taking advantage of slight successive variations;she can never take a great and sudden leap, but must advance by the shortand sure, though slow steps.

ORGANS OF LITTLE APPARENT IMPORTANCE, AS AFFECTED BY NATURAL SELECTION.

As natural selection acts by life and death, by the survival of thefittest, and by the destruction of the less well-fitted individuals, I havesometimes felt great difficulty in understanding the origin or formation ofparts of little importance; almost as great, though of a very differentkind, as in the case of the most perfect and complex organs.

In the first place, we are much too ignorant in regard to the whole economyof any one organic being to say what slight modifications would be ofimportance or not. In a former chapter I have given instances of verytrifling characters, such as the down on fruit and the colour of its flesh,the colour of the skin and hair of quadrupeds, which, from being correlatedwith constitutional differences, or from determining the attacks ofinsects, might assuredly be acted on by natural selection. The tail of thegiraffe looks like an artificially constructed fly-flapper; and it seems atfirst incredible that this could have been adapted for its present purposeby successive slight modifications, each better and better fitted, for sotrifling an object as to drive away flies; yet we should pause before beingtoo positive even in this case, for we know that the distribution andexistence of cattle and other animals in South America absolutely depend ontheir power of resisting the attacks of insects: so that individuals whichcould by any means defend themselves from these small enemies, would beable to range into new pastures and thus gain a great advantage. It is notthat the larger quadrupeds are actually destroyed (except in some rarecases) by flies, but they are incessantly harassed and their strengthreduced, so that they are more subject to disease, or not so well enabledin a coming dearth to search for food, or to escape from beasts of prey.

Organs now of trifling importance have probably in some cases been of highimportance to an early progenitor, and, after having been slowly perfectedat a former period, have been transmitted to existing species in nearly thesame state, although now of very slight use; but any actually injuriousdeviations in their structure would of course have been checked by naturalselection. Seeing how important an organ of locomotion the tail is in mostaquatic animals, its general presence and use for many purposes in so manyland animals, which in their lungs or modified swim-bladders betray theiraquatic origin, may perhaps be thus accounted for. A well-developed tailhaving been formed in an aquatic animal, it might subsequently come to beworked in for all sorts of purposes, as a fly-flapper, an organ ofprehension, or as an aid in turning, as in the case of the dog, though theaid in this latter respect must be slight, for the hare, with hardly anytail, can double still more quickly.

In the second place, we may easily err in attributing importance tocharacters, and in believing that they have been developed through naturalselection. We must by no means overlook the effects of the definite actionof changed conditions of life, of so-called spontaneous variations, whichseem to depend in a quite subordinate degree on the nature of theconditions, of the tendency to reversion to long-lost characters, of thecomplex laws of growth, such as of correlation, comprehension, of thepressure of one part on another, etc., and finally of sexual selection, bywhich characters of use to one sex are often gained and then transmittedmore or less perfectly to the other sex, though of no use to the sex. Butstructures thus indirectly gained, although at first of no advantage to aspecies, may subsequently have been taken advantage of by its modifieddescendants, under new conditions of life and newly acquired habits.

If green woodpeckers alone had existed, and we did not know that there weremany black and pied kinds, I dare say that we should have thought that thegreen colour was a beautiful adaptation to conceal this tree-frequentingbird from its enemies; and consequently that it was a character ofimportance, and had been acquired through natural selection; as it is, thecolour is probably in chief part due to sexual selection. A trailing palmin the Malay Archipelago climbs the loftiest trees by the aid ofexquisitely constructed hooks clustered around the ends of the branches,and this contrivance, no doubt, is of the highest service to the plant; butas we see nearly similar hooks on many trees which are not climbers, andwhich, as there is reason to believe from the distribution of the thorn-bearing species in Africa and South America, serve as a defence againstbrowsing quadrupeds, so the spikes on the palm may at first have beendeveloped for this object, and subsequently have been improved and takenadvantage of by the plant, as it underwent further modification and becamea climber. The naked skin on the head of a vulture is generally consideredas a direct adaptation for wallowing in putridity; and so it may be, or itmay possibly be due to the direct action of putrid matter; but we should bevery cautious in drawing any such inference, when we see that the skin onthe head of the clean-feeding male turkey is likewise naked. The suturesin the skulls of young mammals have been advanced as a beautiful adaptationfor aiding parturition, and no doubt they facilitate, or may beindispensable for this act; but as sutures occur in the skulls of youngbirds and reptiles, which have only to escape from a broken egg, we mayinfer that this structure has arisen from the laws of growth, and has beentaken advantage of in the parturition of the higher animals.

We are profoundly ignorant of the cause of each slight variation orindividual difference; and we are immediately made conscious of this byreflecting on the differences between the breeds of our domesticatedanimals in different countries, more especially in the less civilizedcountries, where there has been but little methodical selection. Animalskept by savages in different countries often have to struggle for their ownsubsistence, and are exposed to a certain extent to natural selection, andindividuals with slightly different constitutions would succeed best underdifferent climates. With cattle susceptibility to the attacks of flies iscorrelated with colour, as is the liability to be poisoned by certainplants; so that even colour would be thus subjected to the action ofnatural selection. Some observers are convinced that a damp climateaffects the growth of the hair, and that with the hair the horns arecorrelated. Mountain breeds always differ from lowland breeds; and amountainous country would probably affect the hind limbs from exercisingthem more, and possibly even the form of the pelvis; and then by the law ofhomologous variation, the front limbs and the head would probably beaffected. The shape, also, of the pelvis might affect by pressure theshape of certain parts of the young in the womb. The laborious breathingnecessary in high regions tends, as we have good reason to believe, toincrease the size of the chest; and again correlation would come into play. The effects of lessened exercise, together with abundant food, on the wholeorganisation is probably still more important, and this, as H. vonNathusius has lately shown in his excellent Treatise, is apparently onechief cause of the great modification which the breeds of swine haveundergone. But we are far too ignorant to speculate on the relativeimportance of the several known and unknown causes of variation; and I havemade these remarks only to show that, if we are unable to account for thecharacteristic differences of our several domestic breeds, whichnevertheless are generally admitted to have arisen through ordinarygeneration from one or a few parent-stocks, we ought not to lay too muchstress on our ignorance of the precise cause of the slight analogousdifferences between true species.

UTILITARIAN DOCTRINE, HOW FAR TRUE: BEAUTY, HOW ACQUIRED.

The foregoing remarks lead me to say a few words on the protest lately madeby some naturalists against the utilitarian doctrine that every detail ofstructure has been produced for the good of its possessor. They believethat many structures have been created for the sake of beauty, to delightman or the Creator (but this latter point is beyond the scope of scientificdiscussion), or for the sake of mere variety, a view already discussed. Such doctrines, if true, would be absolutely fatal to my theory. I fullyadmit that many structures are now of no direct use to their possessors,and may never have been of any use to their progenitors; but this does notprove that they were formed solely for beauty or variety. No doubt thedefinite action of changed conditions, and the various causes ofmodifications, lately specified, have all produced an effect, probably agreat effect, independently of any advantage thus gained. But a still moreimportant consideration is that the chief part of the organisation of everyliving creature is due to inheritance; and consequently, though each beingassuredly is well fitted for its place in nature, many structures have nowno very close and direct relation to present habits of life. Thus, we canhardly believe that the webbed feet of the upland goose, or of the frigate-bird, are of special use to these birds; we cannot believe that the similarbones in the arm of the monkey, in the fore leg of the horse, in the wingof the bat, and in the flipper of the seal, are of special use to theseanimals. We may safely attribute these structures to inheritance. Butwebbed feet no doubt were as useful to the progenitor of the upland gooseand of the frigate-bird, as they now are to the most aquatic of livingbirds. So we may believe that the progenitor of the seal did not possess aflipper, but a foot with five toes fitted for walking or grasping; and wemay further venture to believe that the several bones in the limbs of themonkey, horse and bat, were originally developed, on the principle ofutility, probably through the reduction of more numerous bones in the finof some ancient fish-like progenitor of the whole class. It is scarcelypossible to decide how much allowance ought to be made for such causes ofchange, as the definite action of external conditions, so-calledspontaneous variations, and the complex laws of growth; but with theseimportant exceptions, we may conclude that the structure of every livingcreature either now is, or was formerly, of some direct or indirect use toits possessor.

With respect to the belief that organic beings have been created beautifulfor the delight of man--a belief which it has been pronounced is subversiveof my whole theory--I may first remark that the sense of beauty obviouslydepends on the nature of the mind, irrespective of any real quality in theadmired object; and that the idea of what is beautiful, is not innate orunalterable. We see this, for instance, in the men of different racesadmiring an entirely different standard of beauty in their women. Ifbeautiful objects had been created solely for man's gratification, it oughtto be shown that before man appeared there was less beauty on the face ofthe earth than since he came on the stage. Were the beautiful volute andcone shells of the Eocene epoch, and the gracefully sculptured ammonites ofthe Secondary period, created that man might ages afterwards admire them inhis cabinet? Few objects are more beautiful than the minute siliceouscases of the diatomaceae: were these created that they might be examinedand admired under the higher powers of the microscope? The beauty in thislatter case, and in many others, is apparently wholly due to symmetry ofgrowth. Flowers rank among the most beautiful productions of nature; butthey have been rendered conspicuous in contrast with the green leaves, andin consequence at the same time beautiful, so that they may be easilyobserved by insects. I have come to this conclusion from finding it aninvariable rule that when a flower is fertilised by the wind it never has agaily-coloured corolla. Several plants habitually produce two kinds offlowers; one kind open and coloured so as to attract insects; the otherclosed, not coloured, destitute of nectar, and never visited by insects. Hence, we may conclude that, if insects had not been developed on the faceof the earth, our plants would not have been decked with beautiful flowers,but would have produced only such poor flowers as we see on our fir, oak,nut and ash trees, on grasses, spinach, docks and nettles, which are allfertilised through the agency of the wind. A similar line of argumentholds good with fruits; that a ripe strawberry or cherry is as pleasing tothe eye as to the palate--that the gaily-coloured fruit of the spindle-woodtree and the scarlet berries of the holly are beautiful objects--will beadmitted by everyone. But this beauty serves merely as a guide to birdsand beasts, in order that the fruit may be devoured and the matured seedsdisseminated. I infer that this is the case from having as yet found noexception to the rule that seeds are always thus disseminated when embeddedwithin a fruit of any kind (that is within a fleshy or pulpy envelope), ifit be coloured of any brilliant tint, or rendered conspicuous by beingwhite or black.

On the other hand, I willingly admit that a great number of male animals,as all our most gorgeous birds, some fishes, reptiles, and mammals, and ahost of magnificently coloured butterflies, have been rendered beautifulfor beauty's sake. But this has been effected through sexual selection,that is, by the more beautiful males having been continually preferred bythe females, and not for the delight of man. So it is with the music ofbirds. We may infer from all this that a nearly similar taste forbeautiful colours and for musical sounds runs through a large part of theanimal kingdom. When the female is as beautifully coloured as the male,which is not rarely the case with birds and butterflies, the causeapparently lies in the colours acquired through sexual selection havingbeen transmitted to both sexes, instead of to the males alone. How thesense of beauty in its simplest form--that is, the reception of a peculiarkind of pleasure from certain colours, forms and sounds--was firstdeveloped in the mind of man and of the lower animals, is a very obscuresubject. The same sort of difficulty is presented if we enquire how it isthat certain flavours and odours give pleasure, and others displeasure. Habit in all these cases appears to have come to a certain extent intoplay; but there must be some fundamental cause in the constitution of thenervous system in each species.

Natural selection cannot possibly produce any modification in a speciesexclusively for the good of another species; though throughout nature onespecies incessantly takes advantage of, and profits by the structures ofothers. But natural selection can and does often produce structures forthe direct injury of other animals, as we see in the fang of the adder, andin the ovipositor of the ichneumon, by which its eggs are deposited in theliving bodies of other insects. If it could be proved that any part of thestructure of any one species had been formed for the exclusive good ofanother species, it would annihilate my theory, for such could not havebeen produced through natural selection. Although many statements may befound in works on natural history to this effect, I cannot find even onewhich seems to me of any weight. It is admitted that the rattlesnake has apoison-fang for its own defence and for the destruction of its prey; butsome authors suppose that at the same time it is furnished with a rattlefor its own injury, namely, to warn its prey. I would almost as soonbelieve that the cat curls the end of its tail when preparing to spring, inorder to warn the doomed mouse. It is a much more probable view that therattlesnake uses its rattle, the cobra expands its frill and the puff-adderswells while hissing so loudly and harshly, in order to alarm the manybirds and beasts which are known to attack even the most venomous species. Snakes act on the same principle which makes the hen ruffle her feathersand expand her wings when a dog approaches her chickens. But I have notspace here to enlarge on the many ways by which animals endeavour tofrighten away their enemies.

Natural selection will never produce in a being any structure moreinjurious than beneficial to that being, for natural selection acts solelyby and for the good of each. No organ will be formed, as Paley hasremarked, for the purpose of causing pain or for doing an injury to itspossessor. If a fair balance be struck between the good and evil caused byeach part, each will be found on the whole advantageous. After the lapseof time, under changing conditions of life, if any part comes to beinjurious, it will be modified; or if it be not so, the being will becomeextinct, as myriads have become extinct.

Natural selection tends only to make each organic being as perfect as, orslightly more perfect than the other inhabitants of the same country withwhich it comes into competition. And we see that this is the standard ofperfection attained under nature. The endemic productions of New Zealand,for instance, are perfect, one compared with another; but they are nowrapidly yielding before the advancing legions of plants and animalsintroduced from Europe. Natural selection will not produce absoluteperfection, nor do we always meet, as far as we can judge, with this highstandard under nature. The correction for the aberration of light is saidby Muller not to be perfect even in that most perfect organ, the human eye. Helmholtz, whose judgment no one will dispute, after describing in thestrongest terms the wonderful powers of the human eye, adds theseremarkable words: "That which we have discovered in the way of inexactnessand imperfection in the optical machine and in the image on the retina, isas nothing in comparison with the incongruities which we have just comeacross in the domain of the sensations. One might say that nature hastaken delight in accumulating contradictions in order to remove allfoundation from the theory of a pre-existing harmony between the externaland internal worlds." If our reason leads us to admire with enthusiasm amultitude of inimitable contrivances in nature, this same reason tells us,though we may easily err on both sides, that some other contrivances areless perfect. Can we consider the sting of the bee as perfect, which, whenused against many kinds of enemies, cannot be withdrawn, owing to thebackward serratures, and thus inevitably causes the death of the insect bytearing out its viscera?

If we look at the sting of the bee, as having existed in a remoteprogenitor, as a boring and serrated instrument, like that in so manymembers of the same great order, and that it has since been modified butnot perfected for its present purpose, with the poison originally adaptedfor some other object, such as to produce galls, since intensified, we canperhaps understand how it is that the use of the sting should so oftencause the insect's own death: for if on the whole the power of stinging beuseful to the social community, it will fulfil all the requirements ofnatural selection, though it may cause the death of some few members. Ifwe admire the truly wonderful power of scent by which the males of manyinsects find their females, can we admire the production for this singlepurpose of thousands of drones, which are utterly useless to the communityfor any other purpose, and which are ultimately slaughtered by theirindustrious and sterile sisters? It may be difficult, but we ought toadmire the savage instinctive hatred of the queen-bee, which urges her todestroy the young queens, her daughters, as soon as they are born, or toperish herself in the combat; for undoubtedly this is for the good of thecommunity; and maternal love or maternal hatred, though the latterfortunately is most rare, is all the same to the inexorable principles ofnatural selection. If we admire the several ingenious contrivances bywhich orchids and many other plants are fertilised through insect agency,can we consider as equally perfect the elaboration of dense clouds ofpollen by our fir-trees, so that a few granules may be wafted by chance onto the ovules?

SUMMARY: THE LAW OF UNITY OF TYPE AND OF THE CONDITIONS OF EXISTENCEEMBRACED BY THE THEORY OF NATURAL SELECTION.

We have in this chapter discussed some of the difficulties and objectionswhich may be urged against the theory. Many of them are serious; but Ithink that in the discussion light has been thrown on several facts, whichon the belief of independent acts of creation are utterly obscure. We haveseen that species at any one period are not indefinitely variable, and arenot linked together by a multitude of intermediate gradations, partlybecause the process of natural selection is always very slow, and at anyone time acts only on a few forms; and partly because the very process ofnatural selection implies the continual supplanting and extinction ofpreceding and intermediate gradations. Closely allied species, now livingon a continuous area, must often have been formed when the area was notcontinuous, and when the conditions of life did not insensibly graduateaway from one part to another. When two varieties are formed in twodistricts of a continuous area, an intermediate variety will often beformed, fitted for an intermediate zone; but from reasons assigned, theintermediate variety will usually exist in lesser numbers than the twoforms which it connects; consequently the two latter, during the course offurther modification, from existing in greater numbers, will have a greatadvantage over the less numerous intermediate variety, and will thusgenerally succeed in supplanting and exterminating it.

We have seen in this chapter how cautious we should be in concluding thatthe most different habits of life could not graduate into each other; thata bat, for instance, could not have been formed by natural selection froman animal which at first only glided through the air.

We have seen that a species under new conditions of life may change itshabits, or it may have diversified habits, with some very unlike those ofits nearest congeners. Hence we can understand, bearing in mind that eachorganic being is trying to live wherever it can live, how it has arisenthat there are upland geese with webbed feet, ground woodpeckers, divingthrushes, and petrels with the habits of auks.

Although the belief that an organ so perfect as the eye could have beenformed by natural selection, is enough to stagger any one; yet in the caseof any organ, if we know of a long series of gradations in complexity, eachgood for its possessor, then under changing conditions of life, there is nological impossibility in the acquirement of any conceivable degree ofperfection through natural selection. In the cases in which we know of nointermediate or transitional states, we should be extremely cautious inconcluding that none can have existed, for the metamorphoses of many organsshow what wonderful changes in function are at least possible. Forinstance, a swim-bladder has apparently been converted into anair-breathing lung. The same organ having performed simultaneously verydifferent functions, and then having been in part or in whole specialisedfor one function; and two distinct organs having performed at the same timethe same function, the one having been perfected whilst aided by the other,must often have largely facilitated transitions.

We have seen that in two beings widely remote from each other in thenatural scale, organs serving for the same purpose and in externalappearance closely similar may have been separately and independentlyformed; but when such organs are closely examined, essential differences intheir structure can almost always be detected; and this naturally followsfrom the principle of natural selection. On the other hand, the commonrule throughout nature is infinite diversity of structure for gaining thesame end; and this again naturally follows from the same great principle.

In many cases we are far too ignorant to be enabled to assert that a partor organ is so unimportant for the welfare of a species, that modificationsin its structure could not have been slowly accumulated by means of naturalselection. In many other cases, modifications are probably the directresult of the laws of variation or of growth, independently of any goodhaving been thus gained. But even such structures have often, as we mayfeel assured, been subsequently taken advantage of, and still furthermodified, for the good of species under new conditions of life. We may,also, believe that a part formerly of high importance has frequently beenretained (as the tail of an aquatic animal by its terrestrial descendants),though it has become of such small importance that it could not, in itspresent state, have been acquired by means of natural selection.

Natural selection can produce nothing in one species for the exclusive goodor injury of another; though it may well produce parts, organs, andexcretions highly useful or even indispensable, or highly injurious toanother species, but in all cases at the same time useful to the possessor.In each well-stocked country natural selection acts through the competitionof the inhabitants and consequently leads to success in the battle forlife, only in accordance with the standard of that particular country. Hence the inhabitants of one country, generally the smaller one, oftenyield to the inhabitants of another and generally the larger country. Forin the larger country there will have existed more individuals, and morediversified forms, and the competition will have been severer, and thus thestandard of perfection will have been rendered higher. Natural selectionwill not necessarily lead to absolute perfection; nor, as far as we canjudge by our limited faculties, can absolute perfection be everywherepredicated.

On the theory of natural selection we can clearly understand the fullmeaning of that old canon in natural history, "Natura non facit saltum." This canon, if we look to the present inhabitants alone of the world, isnot strictly correct; but if we include all those of past times, whetherknown or unknown, it must on this theory be strictly true.

It is generally acknowledged that all organic beings have been formed ontwo great laws--Unity of Type, and the Conditions of Existence. By unityof type is meant that fundamental agreement in structure which we see inorganic beings of the same class, and which is quite independent of theirhabits of life. On my theory, unity of type is explained by unity ofdescent. The expression of conditions of existence, so often insisted onby the illustrious Cuvier, is fully embraced by the principle of naturalselection. For natural selection acts by either now adapting the varyingparts of each being to its organic and inorganic conditions of life; or byhaving adapted them during past periods of time: the adaptations beingaided in many cases by the increased use or disuse of parts, being affectedby the direct action of external conditions of life, and subjected in allcases to the several laws of growth and variation. Hence, in fact, the lawof the Conditions of Existence is the higher law; as it includes, throughthe inheritance of former variations and adaptations, that of Unity ofType.