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Recapitulation of the objections to the theory of Natural Selection --Recapitulation of the general and special circumstances in its favour --Causes of the general belief in the immutability of species -- How far thetheory of Natural Selection may be extended -- Effects of its adoption onthe study of Natural History -- Concluding remarks.

As this whole volume is one long argument, it may be convenient to thereader to have the leading facts and inferences briefly recapitulated.

That many and serious objections may be advanced against the theory ofdescent with modification through variation and natural selection, I do notdeny. I have endeavoured to give to them their full force. Nothing atfirst can appear more difficult to believe than that the more complexorgans and instincts have been perfected, not by means superior to, thoughanalogous with, human reason, but by the accumulation of innumerable slightvariations, each good for the individual possessor. Nevertheless, thisdifficulty, though appearing to our imagination insuperably great, cannotbe considered real if we admit the following propositions, namely, that allparts of the organisation and instincts offer, at least individualdifferences--that there is a struggle for existence leading to thepreservation of profitable deviations of structure or instinct--and,lastly, that gradations in the state of perfection of each organ may haveexisted, each good of its kind. The truth of these propositions cannot, Ithink, be disputed.

It is, no doubt, extremely difficult even to conjecture by what gradationsmany structures have been perfected, more especially among broken andfailing groups of organic beings, which have suffered much extinction; butwe see so many strange gradations in nature, that we ought to be extremelycautious in saying that any organ or instinct, or any whole structure,could not have arrived at its present state by many graduated steps. Thereare, it must be admitted, cases of special difficulty opposed to the theoryof natural selection; and one of the most curious of these is the existence in the same community of two or three defined castes of workers or sterilefemale ants; but I have attempted to show how these difficulties can bemastered.

With respect to the almost universal sterility of species when firstcrossed, which forms so remarkable a contrast with the almost universalfertility of varieties when crossed, I must refer the reader to therecapitulation of the facts given at the end of the ninth chapter, whichseem to me conclusively to show that this sterility is no more a specialendowment than is the incapacity of two distinct kinds of trees to begrafted together; but that it is incidental on differences confined to thereproductive systems of the intercrossed species. We see the truth of thisconclusion in the vast difference in the results of crossing the same twospecies reciprocally--that is, when one species is first used as the fatherand then as the mother. Analogy from the consideration of dimorphic andtrimorphic plants clearly leads to the same conclusion, for when the formsare illegitimately united, they yield few or no seed, and their offspringare more or less sterile; and these forms belong to the same undoubtedspecies, and differ from each other in no respect except in theirreproductive organs and functions.

Although the fertility of varieties when intercrossed, and of their mongreloffspring, has been asserted by so many authors to be universal, thiscannot be considered as quite correct after the facts given on the highauthority of Gartner and Kolreuter. Most of the varieties which have beenexperimented on have been produced under domestication; and asdomestication (I do not mean mere confinement) almost certainly tends toeliminate that sterility which, judging from analogy, would have affectedthe parent-species if intercrossed, we ought not to expect thatdomestication would likewise induce sterility in their modified descendantswhen crossed. This elimination of sterility apparently follows from thesame cause which allows our domestic animals to breed freely underdiversified circumstances; and this again apparently follows from theirhaving been gradually accustomed to frequent changes in their conditions oflife.

A double and parallel series of facts seems to throw much light on thesterility of species, when first crossed, and of their hybrid offspring. On the one side, there is good reason to believe that slight changes in theconditions of life give vigour and fertility to all organic beings. Weknow also that a cross between the distinct individuals of the samevariety, and between distinct varieties, increases the number of theiroffspring, and certainly gives to them increased size and vigour. This ischiefly owing to the forms which are crossed having been exposed tosomewhat different conditions of life; for I have ascertained by alabourious series of experiments that if all the individuals of the samevariety be subjected during several generations to the same conditions, thegood derived from crossing is often much diminished or wholly disappears. This is one side of the case. On the other side, we know that specieswhich have long been exposed to nearly uniform conditions, when they aresubjected under confinement to new and greatly changed conditions, eitherperish, or if they survive, are rendered sterile, though retaining perfecthealth. This does not occur, or only in a very slight degree, with ourdomesticated productions, which have long been exposed to fluctuatingconditions. Hence when we find that hybrids produced by a cross betweentwo distinct species are few in number, owing to their perishing soon afterconception or at a very early age, or if surviving that they are renderedmore or less sterile, it seems highly probable that this result is due totheir having been in fact subjected to a great change in their conditionsof life, from being compounded of two distinct organisations. He who willexplain in a definite manner why, for instance, an elephant or a fox willnot breed under confinement in its native country, whilst the domestic pigor dog will breed freely under the most diversified conditions, will at thesame time be able to give a definite answer to the question why twodistinct species, when crossed, as well as their hybrid offspring, aregenerally rendered more or less sterile, while two domesticated varietieswhen crossed and their mongrel offspring are perfectly fertile.

Turning to geographical distribution, the difficulties encountered on thetheory of descent with modification are serious enough. All theindividuals of the same species, and all the species of the same genus, oreven higher group, are descended from common parents; and therefore, inhowever distant and isolated parts of the world they may now be found, theymust in the course of successive generations have travelled from some onepoint to all the others. We are often wholly unable even to conjecture howthis could have been effected. Yet, as we have reason to believe that somespecies have retained the same specific form for very long periods of time,immensely long as measured by years, too much stress ought not to be laidon the occasional wide diffusion of the same species; for during very longperiods there will always have been a good chance for wide migration bymany means. A broken or interrupted range may often be accounted for bythe extinction of the species in the intermediate regions. It cannot bedenied that we are as yet very ignorant as to the full extent of thevarious climatical and geographical changes which have affected the earthduring modern periods; and such changes will often have facilitatedmigration. As an example, I have attempted to show how potent has been theinfluence of the Glacial period on the distribution of the same and ofallied species throughout the world. We are as yet profoundly ignorant ofthe many occasional means of transport. With respect to distinct speciesof the same genus, inhabiting distant and isolated regions, as the processof modification has necessarily been slow, all the means of migration willhave been possible during a very long period; and consequently thedifficulty of the wide diffusion of the species of the same genus is insome degree lessened.

As according to the theory of natural selection an interminable number ofintermediate forms must have existed, linking together all the species ineach group by gradations as fine as our existing varieties, it may beasked, Why do we not see these linking forms all around us? Why are notall organic beings blended together in an inextricable chaos? With respectto existing forms, we should remember that we have no right to expect(excepting in rare cases) to discover DIRECTLY connecting links betweenthem, but only between each and some extinct and supplanted form. Even ona wide area, which has during a long period remained continuous, and ofwhich the climatic and other conditions of life change insensibly inproceeding from a district occupied by one species into another districtoccupied by a closely allied species, we have no just right to expect oftento find intermediate varieties in the intermediate zones. For we havereason to believe that only a few species of a genus ever undergo change;the other species becoming utterly extinct and leaving no modified progeny.Of the species which do change, only a few within the same country changeat the same time; and all modifications are slowly effected. I have alsoshown that the intermediate varieties which probably at first existed inthe intermediate zones, would be liable to be supplanted by the alliedforms on either hand; for the latter, from existing in greater numbers,would generally be modified and improved at a quicker rate than theintermediate varieties, which existed in lesser numbers; so that theintermediate varieties would, in the long run, be supplanted andexterminated.

On this doctrine of the extermination of an infinitude of connecting links,between the living and extinct inhabitants of the world, and at eachsuccessive period between the extinct and still older species, why is notevery geological formation charged with such links? Why does not everycollection of fossil remains afford plain evidence of the gradation andmutation of the forms of life? Although geological research hasundoubtedly revealed the former existence of many links, bringing numerousforms of life much closer together, it does not yield the infinitely manyfine gradations between past and present species required on the theory,and this is the most obvious of the many objections which may be urgedagainst it. Why, again, do whole groups of allied species appear, thoughthis appearance is often false, to have come in suddenly on the successivegeological stages? Although we now know that organic beings appeared onthis globe, at a period incalculably remote, long before the lowest bed ofthe Cambrian system was deposited, why do we not find beneath this systemgreat piles of strata stored with the remains of the progenitors of theCambrian fossils? For on the theory, such strata must somewhere have beendeposited at these ancient and utterly unknown epochs of the world'shistory.

I can answer these questions and objections only on the supposition thatthe geological record is far more imperfect than most geologists believe. The number of specimens in all our museums is absolutely as nothingcompared with the countless generations of countless species which havecertainly existed. The parent form of any two or more species would not bein all its characters directly intermediate between its modified offspring,any more than the rock-pigeon is directly intermediate in crop and tailbetween its descendants, the pouter and fantail pigeons. We should not beable to recognise a species as the parent of another and modified species,if we were to examine the two ever so closely, unless we possessed most ofthe intermediate links; and owing to the imperfection of the geologicalrecord, we have no just right to expect to find so many links. If two orthree, or even more linking forms were discovered, they would simply beranked by many naturalists as so many new species, more especially if foundin different geological substages, let their differences be ever so slight. Numerous existing doubtful forms could be named which are probablyvarieties; but who will pretend that in future ages so many fossil linkswill be discovered, that naturalists will be able to decide whether or notthese doubtful forms ought to be called varieties? Only a small portion ofthe world has been geologically explored. Only organic beings of certainclasses can be preserved in a fossil condition, at least in any greatnumber. Many species when once formed never undergo any further change butbecome extinct without leaving modified descendants; and the periods duringwhich species have undergone modification, though long as measured byyears, have probably been short in comparison with the periods during whichthey retained the same form. It is the dominant and widely ranging specieswhich vary most frequently and vary most, and varieties are often at firstlocal--both causes rendering the discovery of intermediate links in any oneformation less likely. Local varieties will not spread into other anddistant regions until they are considerably modified and improved; and whenthey have spread, and are discovered in a geological formation, they appearas if suddenly created there, and will be simply classed as new species. Most formations have been intermittent in their accumulation; and theirduration has probably been shorter than the average duration of specificforms. Successive formations are in most cases separated from each otherby blank intervals of time of great length, for fossiliferous formationsthick enough to resist future degradation can, as a general rule, beaccumulated only where much sediment is deposited on the subsiding bed ofthe sea. During the alternate periods of elevation and of stationary levelthe record will generally be blank. During these latter periods there willprobably be more variability in the forms of life; during periods ofsubsidence, more extinction.

With respect to the absence of strata rich in fossils beneath the Cambrianformation, I can recur only to the hypothesis given in the tenth chapter;namely, that though our continents and oceans have endured for an enormousperiod in nearly their present relative positions, we have no reason toassume that this has always been the case; consequently formations mucholder than any now known may lie buried beneath the great oceans. Withrespect to the lapse of time not having been sufficient since our planetwas consolidated for the assumed amount of organic change, and thisobjection, as urged by Sir William Thompson, is probably one of the gravestas yet advanced, I can only say, firstly, that we do not know at what ratespecies change, as measured by years, and secondly, that many philosophersare not as yet willing to admit that we know enough of the constitution ofthe universe and of the interior of our globe to speculate with safety onits past duration.

That the geological record is imperfect all will admit; but that it isimperfect to the degree required by our theory, few will be inclined toadmit. If we look to long enough intervals of time, geology plainlydeclares that species have all changed; and they have changed in the mannerrequired by the theory, for they have changed slowly and in a graduatedmanner. We clearly see this in the fossil remains from consecutiveformations invariably being much more closely related to each other thanare the fossils from widely separated formations.

Such is the sum of the several chief objections and difficulties which mayjustly be urged against the theory; and I have now briefly recapitulatedthe answers and explanations which, as far as I can see, may be given. Ihave felt these difficulties far too heavily during many years to doubttheir weight. But it deserves especial notice that the more importantobjections relate to questions on which we are confessedly ignorant; nor dowe know how ignorant we are. We do not know all the possible transitionalgradations between the simplest and the most perfect organs; it cannot bepretended that we know all the varied means of Distribution during the longlapse of years, or that we know how imperfect is the Geological Record. Serious as these several objections are, in my judgment they are by nomeans sufficient to overthrow the theory of descent with subsequentmodification.

Now let us turn to the other side of the argument. Under domestication wesee much variability, caused, or at least excited, by changed conditions oflife; but often in so obscure a manner, that we are tempted to consider thevariations as spontaneous. Variability is governed by many complex laws,by correlated growth, compensation, the increased use and disuse of parts,and the definite action of the surrounding conditions. There is muchdifficulty in ascertaining how largely our domestic productions have beenmodified; but we may safely infer that the amount has been large, and thatmodifications can be inherited for long periods. As long as the conditionsof life remain the same, we have reason to believe that a modification,which has already been inherited for many generations, may continue to beinherited for an almost infinite number of generations. On the other handwe have evidence that variability, when it has once come into play, doesnot cease under domestication for a very long period; nor do we know thatit ever ceases, for new varieties are still occasionally produced by ouroldest domesticated productions.

Variability is not actually caused by man; he only unintentionally exposesorganic beings to new conditions of life and then nature acts on theorganisation and causes it to vary. But man can and does select thevariations given to him by nature, and thus accumulates them in any desiredmanner. He thus adapts animals and plants for his own benefit or pleasure. He may do this methodically, or he may do it unconsciously by preservingthe individuals most useful or pleasing to him without any intention ofaltering the breed. It is certain that he can largely influence thecharacter of a breed by selecting, in each successive generation,individual differences so slight as to be inappreciable except by aneducated eye. This unconscious process of selection has been the greatagency in the formation of the most distinct and useful domestic breeds. That many breeds produced by man have to a large extent the character ofnatural species, is shown by the inextricable doubts whether many of themare varieties or aboriginally distinct species.

There is no reason why the principles which have acted so efficiently underdomestication should not have acted under nature. In the survival offavoured individuals and races, during the constantly recurrent Strugglefor Existence, we see a powerful and ever-acting form of Selection. Thestruggle for existence inevitably follows from the high geometrical ratioof increase which is common to all organic beings. This high rate ofincrease is proved by calculation--by the rapid increase of many animalsand plants during a succession of peculiar seasons, and when naturalised innew countries. More individuals are born than can possibly survive. Agrain in the balance may determine which individuals shall live and whichshall die--which variety or species shall increase in number, and whichshall decrease, or finally become extinct. As the individuals of the samespecies come in all respects into the closest competition with each other,the struggle will generally be most severe between them; it will be almostequally severe between the varieties of the same species, and next inseverity between the species of the same genus. On the other hand thestruggle will often be severe between beings remote in the scale of nature. The slightest advantage in certain individuals, at any age or during anyseason, over those with which they come into competition, or betteradaptation in however slight a degree to the surrounding physicalconditions, will, in the long run, turn the balance.

With animals having separated sexes, there will be in most cases a strugglebetween the males for the possession of the females. The most vigorousmales, or those which have most successfully struggled with theirconditions of life, will generally leave most progeny. But success willoften depend on the males having special weapons or means of defence orcharms; and a slight advantage will lead to victory.

As geology plainly proclaims that each land has undergone great physicalchanges, we might have expected to find that organic beings have variedunder nature, in the same way as they have varied under domestication. Andif there has been any variability under nature, it would be anunaccountable fact if natural selection had not come into play. It hasoften been asserted, but the assertion is incapable of proof, that theamount of variation under nature is a strictly limited quantity. Man,though acting on external characters alone and often capriciously, canproduce within a short period a great result by adding up mere individualdifferences in his domestic productions; and every one admits that speciespresent individual differences. But, besides such differences, allnaturalists admit that natural varieties exist, which are consideredsufficiently distinct to be worthy of record in systematic works. No onehas drawn any clear distinction between individual differences and slightvarieties; or between more plainly marked varieties and subspecies andspecies. On separate continents, and on different parts of the samecontinent, when divided by barriers of any kind, and on outlying islands,what a multitude of forms exist, which some experienced naturalists rank asvarieties, others as geographical races or sub species, and others asdistinct, though closely allied species!

If, then, animals and plants do vary, let it be ever so slightly or slowly,why should not variations or individual differences, which are in any waybeneficial, be preserved and accumulated through natural selection, or thesurvival of the fittest? If man can by patience select variations usefulto him, why, under changing and complex conditions of life, should notvariations useful to nature's living products often arise, and be preservedor selected? What limit can be put to this power, acting during long agesand rigidly scrutinising the whole constitution, structure, and habits ofeach creature, favouring the good and rejecting the bad? I can see nolimit to this power, in slowly and beautifully adapting each form to themost complex relations of life. The theory of natural selection, even ifwe look no further than this, seems to be in the highest degree probable. I have already recapitulated, as fairly as I could, the opposeddifficulties and objections: now let us turn to the special facts andarguments in favour of the theory.

On the view that species are only strongly marked and permanent varieties,and that each species first existed as a variety, we can see why it is thatno line of demarcation can be drawn between species, commonly supposed tohave been produced by special acts of creation, and varieties which areacknowledged to have been produced by secondary laws. On this same view wecan understand how it is that in a region where many species of a genushave been produced, and where they now flourish, these same species shouldpresent many varieties; for where the manufactory of species has beenactive, we might expect, as a general rule, to find it still in action; andthis is the case if varieties be incipient species. Moreover, the speciesof the larger genera, which afford the greater number of varieties orincipient species, retain to a certain degree the character of varieties;for they differ from each other by a less amount of difference than do thespecies of smaller genera. The closely allied species also of a largergenera apparently have restricted ranges, and in their affinities they areclustered in little groups round other species--in both respects resemblingvarieties. These are strange relations on the view that each species wasindependently created, but are intelligible if each existed first as avariety.

As each species tends by its geometrical rate of reproduction to increaseinordinately in number; and as the modified descendants of each specieswill be enabled to increase by as much as they become more diversified inhabits and structure, so as to be able to seize on many and widelydifferent places in the economy of nature, there will be a constanttendency in natural selection to preserve the most divergent offspring ofany one species. Hence during a long-continued course of modification, theslight differences characteristic of varieties of the same species, tend tobe augmented into the greater differences characteristic of the species ofthe same genus. New and improved varieties will inevitably supplant andexterminate the older, less improved and intermediate varieties; and thusspecies are rendered to a large extent defined and distinct objects. Dominant species belonging to the larger groups within each class tend togive birth to new and dominant forms; so that each large group tends tobecome still larger, and at the same time more divergent in character. Butas all groups cannot thus go on increasing in size, for the world would nothold them, the more dominant groups beat the less dominant. This tendencyin the large groups to go on increasing in size and diverging in character,together with the inevitable contingency of much extinction, explains thearrangement of all the forms of life in groups subordinate to groups, allwithin a few great classes, which has prevailed throughout all time. Thisgrand fact of the grouping of all organic beings under what is called theNatural System, is utterly inexplicable on the theory of creation.

As natural selection acts solely by accumulating slight, successive,favourable variations, it can produce no great or sudden modifications; itcan act only by short and slow steps. Hence, the canon of "Natura nonfacit saltum," which every fresh addition to our knowledge tends toconfirm, is on this theory intelligible. We can see why throughout naturethe same general end is gained by an almost infinite diversity of means,for every peculiarity when once acquired is long inherited, and structuresalready modified in many different ways have to be adapted for the samegeneral purpose. We can, in short, see why nature is prodigal in variety,though niggard in innovation. But why this should be a law of nature ifeach species has been independently created no man can explain.

Many other facts are, as it seems to me, explicable on this theory. Howstrange it is that a bird, under the form of a woodpecker, should prey oninsects on the ground; that upland geese, which rarely or never swim, wouldpossess webbed feet; that a thrush-like bird should dive and feed onsub-aquatic insects; and that a petrel should have the habits and structurefitting it for the life of an auk! and so in endless other cases. But onthe view of each species constantly trying to increase in number, withnatural selection always ready to adapt the slowly varying descendants ofeach to any unoccupied or ill-occupied place in nature, these facts ceaseto be strange, or might even have been anticipated.

We can to a certain extent understand how it is that there is so muchbeauty throughout nature; for this may be largely attributed to the agencyof selection. That beauty, according to our sense of it, is not universal,must be admitted by every one who will look at some venomous snakes, atsome fishes, and at certain hideous bats with a distorted resemblance tothe human face. Sexual selection has given the most brilliant colours,elegant patterns, and other ornaments to the males, and sometimes to bothsexes of many birds, butterflies and other animals. With birds it hasoften rendered the voice of the male musical to the female, as well as toour ears. Flowers and fruit have been rendered conspicuous by brilliantcolours in contrast with the green foliage, in order that the flowers maybe easily seen, visited and fertilised by insects, and the seedsdisseminated by birds. How it comes that certain colours, sounds and formsshould give pleasure to man and the lower animals, that is, how the senseof beauty in its simplest form was first acquired, we do not know any morethan how certain odours and flavours were first rendered agreeable.

As natural selection acts by competition, it adapts and improves theinhabitants of each country only in relation to their co-inhabitants; sothat we need feel no surprise at the species of any one country, althoughon the ordinary view supposed to have been created and specially adaptedfor that country, being beaten and supplanted by the naturalisedproductions from another land. Nor ought we to marvel if all thecontrivances in nature be not, as far as we can judge, absolutely perfect;as in the case even of the human eye; or if some of them be abhorrent toour ideas of fitness. We need not marvel at the sting of the bee, whenused against the enemy, causing the bee's own death; at drones beingproduced in such great numbers for one single act, and being thenslaughtered by their sterile sisters; at the astonishing waste of pollen byour fir-trees; at the instinctive hatred of the queen-bee for her ownfertile daughters; at ichneumonidae feeding within the living bodies ofcaterpillars; and at other such cases. The wonder, indeed, is, on thetheory of natural selection, that more cases of the want of absoluteperfection have not been detected.

The complex and little known laws governing the production of varieties arethe same, as far as we can judge, with the laws which have governed theproduction of distinct species. In both cases physical conditions seem tohave produced some direct and definite effect, but how much we cannot say. Thus, when varieties enter any new station, they occasionally assume someof the characters proper to the species of that station. With bothvarieties and species, use and disuse seem to have produced a considerableeffect; for it is impossible to resist this conclusion when we look, forinstance, at the logger-headed duck, which has wings incapable of flight,in nearly the same condition as in the domestic duck; or when we look atthe burrowing tucu-tucu, which is occasionally blind, and then at certainmoles, which are habitually blind and have their eyes covered with skin; orwhen we look at the blind animals inhabiting the dark caves of America andEurope. With varieties and species, correlated variation seems to haveplayed an important part, so that when one part has been modified otherparts have been necessarily modified. With both varieties and species,reversions to long-lost characters occasionally occur. How inexplicable onthe theory of creation is the occasional appearance of stripes on theshoulders and legs of the several species of the horse-genus and of theirhybrids! How simply is this fact explained if we believe that thesespecies are all descended from a striped progenitor, in the same manner asthe several domestic breeds of the pigeon are descended from the blue andbarred rock-pigeon!

On the ordinary view of each species having been independently created, whyshould specific characters, or those by which the species of the same genusdiffer from each other, be more variable than the generic characters inwhich they all agree? Why, for instance, should the colour of a flower bemore likely to vary in any one species of a genus, if the other speciespossess differently coloured flowers, than if all possessed the samecoloured flowers? If species are only well-marked varieties, of which thecharacters have become in a high degree permanent, we can understand thisfact; for they have already varied since they branched off from a commonprogenitor in certain characters, by which they have come to bespecifically distinct from each other; therefore these same characterswould be more likely again to vary than the generic characters which havebeen inherited without change for an immense period. It is inexplicable onthe theory of creation why a part developed in a very unusual manner in onespecies alone of a genus, and therefore, as we may naturally infer, ofgreat importance to that species, should be eminently liable to variation;but, on our view, this part has undergone, since the several speciesbranched off from a common progenitor, an unusual amount of variability andmodification, and therefore we might expect the part generally to be stillvariable. But a part may be developed in the most unusual manner, like thewing of a bat, and yet not be more variable than any other structure, ifthe part be common to many subordinate forms, that is, if it has beeninherited for a very long period; for in this case it will have beenrendered constant by long-continued natural selection.

Glancing at instincts, marvellous as some are, they offer no greaterdifficulty than do corporeal structures on the theory of the naturalselection of successive, slight, but profitable modifications. We can thusunderstand why nature moves by graduated steps in endowing differentanimals of the same class with their several instincts. I have attemptedto show how much light the principle of gradation throws on the admirablearchitectural powers of the hive-bee. Habit no doubt often comes into playin modifying instincts; but it certainly is not indispensable, as we see inthe case of neuter insects, which leave no progeny to inherit the effectsof long-continued habit. On the view of all the species of the same genushaving descended from a common parent, and having inherited much in common,we can understand how it is that allied species, when placed under widelydifferent conditions of life, yet follow nearly the same instincts; why thethrushes of tropical and temperate South America, for instance, line theirnests with mud like our British species. On the view of instincts havingbeen slowly acquired through natural selection, we need not marvel at someinstincts being not perfect and liable to mistakes, and at many instinctscausing other animals to suffer.

If species be only well-marked and permanent varieties, we can at once seewhy their crossed offspring should follow the same complex laws in theirdegrees and kinds of resemblance to their parents--in being absorbed intoeach other by successive crosses, and in other such points--as do thecrossed offspring of acknowledged varieties. This similarity would be astrange fact, if species had been independently created and varieties hadbeen produced through secondary laws.

If we admit that the geological record is imperfect to an extreme degree,then the facts, which the record does give, strongly support the theory ofdescent with modification. New species have come on the stage slowly andat successive intervals; and the amount of change after equal intervals oftime, is widely different in different groups. The extinction of speciesand of whole groups of species, which has played so conspicuous a part inthe history of the organic world, almost inevitably follows from theprinciple of natural selection; for old forms are supplanted by new andimproved forms. Neither single species nor groups of species reappear whenthe chain of ordinary generation is once broken. The gradual diffusion ofdominant forms, with the slow modification of their descendants, causes theforms of life, after long intervals of time, to appear as if they hadchanged simultaneously throughout the world. The fact of the fossilremains of each formation being in some degree intermediate in characterbetween the fossils in the formations above and below, is simply explainedby their intermediate position in the chain of descent. The grand factthat all extinct beings can be classed with all recent beings, naturallyfollows from the living and the extinct being the offspring of commonparents. As species have generally diverged in character during their longcourse of descent and modification, we can understand why it is that themore ancient forms, or early progenitors of each group, so often occupy aposition in some degree intermediate between existing groups. Recent formsare generally looked upon as being, on the whole, higher in the scale oforganisation than ancient forms; and they must be higher, in so far as thelater and more improved forms have conquered the older and less improvedforms in the struggle for life; they have also generally had their organsmore specialised for different functions. This fact is perfectlycompatible with numerous beings still retaining simple and but littleimproved structures, fitted for simple conditions of life; it is likewisecompatible with some forms having retrograded in organisation, by havingbecome at each stage of descent better fitted for new and degraded habitsof life. Lastly, the wonderful law of the long endurance of allied formson the same continent--of marsupials in Australia, of edentata in America,and other such cases--is intelligible, for within the same country theexisting and the extinct will be closely allied by descent.

Looking to geographical distribution, if we admit that there has beenduring the long course of ages much migration from one part of the world toanother, owing to former climatical and geographical changes and to themany occasional and unknown means of dispersal, then we can understand, onthe theory of descent with modification, most of the great leading facts inDistribution. We can see why there should be so striking a parallelism inthe distribution of organic beings throughout space, and in theirgeological succession throughout time; for in both cases the beings havebeen connected by the bond of ordinary generation, and the means ofmodification have been the same. We see the full meaning of the wonderfulfact, which has struck every traveller, namely, that on the same continent,under the most diverse conditions, under heat and cold, on mountain andlowland, on deserts and marshes, most of the inhabitants within each greatclass are plainly related; for they are the descendants of the sameprogenitors and early colonists. On this same principle of formermigration, combined in most cases with modification, we can understand, bythe aid of the Glacial period, the identity of some few plants, and theclose alliance of many others, on the most distant mountains, and in thenorthern and southern temperate zones; and likewise the close alliance ofsome of the inhabitants of the sea in the northern and southern temperatelatitudes, though separated by the whole intertropical ocean. Although twocountries may present physical conditions as closely similar as the samespecies ever require, we need feel no surprise at their inhabitants beingwidely different, if they have been for a long period completely sunderedfrom each other; for as the relation of organism to organism is the mostimportant of all relations, and as the two countries will have receivedcolonists at various periods and in different proportions, from some othercountry or from each other, the course of modification in the two areaswill inevitably have been different.

On this view of migration, with subsequent modification, we see why oceanicislands are inhabited by only few species, but of these, why many arepeculiar or endemic forms. We clearly see why species belonging to thosegroups of animals which cannot cross wide spaces of the ocean, as frogs andterrestrial mammals, do not inhabit oceanic islands; and why, on the otherhand, new and peculiar species of bats, animals which can traverse theocean, are often found on islands far distant from any continent. Suchcases as the presence of peculiar species of bats on oceanic islands andthe absence of all other terrestrial mammals, are facts utterlyinexplicable on the theory of independent acts of creation.

The existence of closely allied representative species in any two areas,implies, on the theory of descent with modification, that the same parent-forms formerly inhabited both areas; and we almost invariably find thatwherever many closely allied species inhabit two areas, some identicalspecies are still common to both. Wherever many closely allied yetdistinct species occur, doubtful forms and varieties belonging to the samegroups likewise occur. It is a rule of high generality that theinhabitants of each area are related to the inhabitants of the nearestsource whence immigrants might have been derived. We see this in thestriking relation of nearly all the plants and animals of the GalapagosArchipelago, of Juan Fernandez, and of the other American islands, to theplants and animals of the neighbouring American mainland; and of those ofthe Cape de Verde Archipelago, and of the other African islands to theAfrican mainland. It must be admitted that these facts receive noexplanation on the theory of creation.

The fact, as we have seen, that all past and present organic beings can bearranged within a few great classes, in groups subordinate to groups, andwith the extinct groups often falling in between the recent groups, isintelligible on the theory of natural selection with its contingencies ofextinction and divergence of character. On these same principles we seehow it is that the mutual affinities of the forms within each class are socomplex and circuitous. We see why certain characters are far moreserviceable than others for classification; why adaptive characters, thoughof paramount importance to the beings, are of hardly any importance inclassification; why characters derived from rudimentary parts, though of noservice to the beings, are often of high classificatory value; and whyembryological characters are often the most valuable of all. The realaffinities of all organic beings, in contradistinction to their adaptiveresemblances, are due to inheritance or community of descent. The NaturalSystem is a genealogical arrangement, with the acquired grades ofdifference, marked by the terms, varieties, species, genera, families,etc.; and we have to discover the lines of descent by the most permanentcharacters, whatever they may be, and of however slight vital importance.

The similar framework of bones in the hand of a man, wing of a bat, fin ofthe porpoise, and leg of the horse--the same number of vertebrae formingthe neck of the giraffe and of the elephant--and innumerable other suchfacts, at once explain themselves on the theory of descent with slow andslight successive modifications. The similarity of pattern in the wing andin the leg of a bat, though used for such different purpose--in the jawsand legs of a crab--in the petals, stamens, and pistils of a flower, islikewise, to a large extent, intelligible on the view of the gradualmodification of parts or organs, which were aboriginally alike in an earlyprogenitor in each of these classes. On the principle of successivevariations not always supervening at an early age, and being inherited at acorresponding not early period of life, we clearly see why the embryos ofmammals, birds, reptiles, and fishes should be so closely similar, and sounlike the adult forms. We may cease marvelling at the embryo of anair-breathing mammal or bird having branchial slits and arteries running inloops, like those of a fish which has to breathe the air dissolved in waterby the aid of well-developed branchiae.

Disuse, aided sometimes by natural selection, will often have reducedorgans when rendered useless under changed habits or conditions of life;and we can understand on this view the meaning of rudimentary organs. Butdisuse and selection will generally act on each creature, when it has cometo maturity and has to play its full part in the struggle for existence,and will thus have little power on an organ during early life; hence theorgan will not be reduced or rendered rudimentary at this early age. Thecalf, for instance, has inherited teeth, which never cut through the gumsof the upper jaw, from an early progenitor having well-developed teeth; andwe may believe, that the teeth in the mature animal were formerly reducedby disuse owing to the tongue and palate, or lips, having becomeexcellently fitted through natural selection to browse without their aid;whereas in the calf, the teeth have been left unaffected, and on theprinciple of inheritance at corresponding ages have been inherited from aremote period to the present day. On the view of each organism with allits separate parts having been specially created, how utterly inexplicableis it that organs bearing the plain stamp of inutility, such as the teethin the embryonic calf or the shrivelled wings under the solderedwing-covers of many beetles, should so frequently occur. Nature may besaid to have taken pains to reveal her scheme of modification, by means ofrudimentary organs, of embryological and homologous structures, but we aretoo blind to understand her meaning.

I have now recapitulated the facts and considerations which have thoroughlyconvinced me that species have been modified, during a long course ofdescent. This has been effected chiefly through the natural selection ofnumerous successive, slight, favourable variations; aided in an importantmanner by the inherited effects of the use and disuse of parts; and in anunimportant manner, that is, in relation to adaptive structures, whetherpast or present, by the direct action of external conditions, and byvariations which seem to us in our ignorance to arise spontaneously. Itappears that I formerly underrated the frequency and value of these latterforms of variation, as leading to permanent modifications of structureindependently of natural selection. But as my conclusions have lately beenmuch misrepresented, and it has been stated that I attribute themodification of species exclusively to natural selection, I may bepermitted to remark that in the first edition of this work, andsubsequently, I placed in a most conspicuous position--namely, at the closeof the Introduction--the following words: "I am convinced that naturalselection has been the main but not the exclusive means of modification." This has been of no avail. Great is the power of steady misrepresentation;but the history of science shows that fortunately this power does not longendure.

It can hardly be supposed that a false theory would explain, in sosatisfactory a manner as does the theory of natural selection, the severallarge classes of facts above specified. It has recently been objected thatthis is an unsafe method of arguing; but it is a method used in judging ofthe common events of life, and has often been used by the greatest naturalphilosophers. The undulatory theory of light has thus been arrived at; andthe belief in the revolution of the earth on its own axis was until latelysupported by hardly any direct evidence. It is no valid objection thatscience as yet throws no light on the far higher problem of the essence ororigin of life. Who can explain what is the essence of the attraction ofgravity? No one now objects to following out the results consequent onthis unknown element of attraction; notwithstanding that Leibnitz formerlyaccused Newton of introducing "occult qualities and miracles intophilosophy."

I see no good reasons why the views given in this volume should shock thereligious feelings of any one. It is satisfactory, as showing howtransient such impressions are, to remember that the greatest discoveryever made by man, namely, the law of the attraction of gravity, was alsoattacked by Leibnitz, "as subversive of natural, and inferentially ofrevealed, religion." A celebrated author and divine has written to me that"he has gradually learned to see that it is just as noble a conception ofthe Deity to believe that He created a few original forms capable of self-development into other and needful forms, as to believe that He required afresh act of creation to supply the voids caused by the action of Hislaws."

Why, it may be asked, until recently did nearly all the most eminent livingnaturalists and geologists disbelieve in the mutability of species? Itcannot be asserted that organic beings in a state of nature are subject tono variation; it cannot be proved that the amount of variation in thecourse of long ages is a limited quantity; no clear distinction has been,or can be, drawn between species and well-marked varieties. It cannot bemaintained that species when intercrossed are invariably sterile andvarieties invariably fertile; or that sterility is a special endowment andsign of creation. The belief that species were immutable productions wasalmost unavoidable as long as the history of the world was thought to be ofshort duration; and now that we have acquired some idea of the lapse oftime, we are too apt to assume, without proof, that the geological recordis so perfect that it would have afforded us plain evidence of the mutationof species, if they had undergone mutation.

But the chief cause of our natural unwillingness to admit that one specieshas given birth to other and distinct species, is that we are always slowin admitting any great changes of which we do not see the steps. Thedifficulty is the same as that felt by so many geologists, when Lyell firstinsisted that long lines of inland cliffs had been formed, and greatvalleys excavated, by the agencies which we still see at work. The mindcannot possibly grasp the full meaning of the term of even a million years;it cannot add up and perceive the full effects of many slight variations,accumulated during an almost infinite number of generations.

Although I am fully convinced of the truth of the views given in thisvolume under the form of an abstract, I by no means expect to convinceexperienced naturalists whose minds are stocked with a multitude of factsall viewed, during a long course of years, from a point of view directlyopposite to mine. It is so easy to hide our ignorance under suchexpressions as the "plan of creation," "unity of design," etc., and tothink that we give an explanation when we only restate a fact. Any onewhose disposition leads him to attach more weight to unexplaineddifficulties than to the explanation of a certain number of facts willcertainly reject the theory. A few naturalists, endowed with muchflexibility of mind, and who have already begun to doubt the immutabilityof species, may be influenced by this volume; but I look with confidence tothe future, to young and rising naturalists, who will be able to view bothsides of the question with impartiality. Whoever is led to believe thatspecies are mutable will do good service by conscientiously expressing hisconviction; for thus only can the load of prejudice by which this subjectis overwhelmed be removed.

Several eminent naturalists have of late published their belief that amultitude of reputed species in each genus are not real species; but thatother species are real, that is, have been independently created. Thisseems to me a strange conclusion to arrive at. They admit that a multitudeof forms, which till lately they themselves thought were special creations,and which are still thus looked at by the majority of naturalists, andwhich consequently have all the external characteristic features of truespecies--they admit that these have been produced by variation, but theyrefuse to extend the same view to other and slightly different forms. Nevertheless, they do not pretend that they can define, or even conjecture,which are the created forms of life, and which are those produced bysecondary laws. They admit variation as a vera causa in one case, theyarbitrarily reject it in another, without assigning any distinction in thetwo cases. The day will come when this will be given as a curiousillustration of the blindness of preconceived opinion. These authors seemno more startled at a miraculous act of creation than at an ordinary birth. But do they really believe that at innumerable periods in the earth'shistory certain elemental atoms have been commanded suddenly to flash intoliving tissues? Do they believe that at each supposed act of creation oneindividual or many were produced? Were all the infinitely numerous kindsof animals and plants created as eggs or seed, or as full grown? and in thecase of mammals, were they created bearing the false marks of nourishmentfrom the mother's womb? Undoubtedly some of these same questions cannot beanswered by those who believe in the appearance or creation of only a fewforms of life or of some one form alone. It has been maintained by severalauthors that it is as easy to believe in the creation of a million beingsas of one; but Maupertuis' philosophical axiom "of least action" leads themind more willingly to admit the smaller number; and certainly we ought notto believe that innumerable beings within each great class have beencreated with plain, but deceptive, marks of descent from a single parent.

As a record of a former state of things, I have retained in the foregoingparagraphs, and elsewhere, several sentences which imply that naturalistsbelieve in the separate creation of each species; and I have been muchcensured for having thus expressed myself. But undoubtedly this was thegeneral belief when the first edition of the present work appeared. Iformerly spoke to very many naturalists on the subject of evolution, andnever once met with any sympathetic agreement. It is probable that somedid then believe in evolution, but they were either silent or expressedthemselves so ambiguously that it was not easy to understand their meaning. Now, things are wholly changed, and almost every naturalist admits thegreat principle of evolution. There are, however, some who still thinkthat species have suddenly given birth, through quite unexplained means, tonew and totally different forms. But, as I have attempted to show, weightyevidence can be opposed to the admission of great and abrupt modifications. Under a scientific point of view, and as leading to further investigation,but little advantage is gained by believing that new forms are suddenlydeveloped in an inexplicable manner from old and widely different forms,over the old belief in the creation of species from the dust of the earth.

It may be asked how far I extend the doctrine of the modification ofspecies. The question is difficult to answer, because the more distinctthe forms are which we consider, by so much the arguments in favour ofcommunity of descent become fewer in number and less in force. But somearguments of the greatest weight extend very far. All the members of wholeclasses are connected together by a chain of affinities, and all can beclassed on the same principle, in groups subordinate to groups. Fossilremains sometimes tend to fill up very wide intervals between existingorders.

Organs in a rudimentary condition plainly show that an early progenitor hadthe organ in a fully developed condition, and this in some cases implies anenormous amount of modification in the descendants. Throughout wholeclasses various structures are formed on the same pattern, and at a veryearly age the embryos closely resemble each other. Therefore I cannotdoubt that the theory of descent with modification embraces all the membersof the same great class or kingdom. I believe that animals are descendedfrom at most only four or five progenitors, and plants from an equal orlesser number.

Analogy would lead me one step further, namely, to the belief that allanimals and plants are descended from some one prototype. But analogy maybe a deceitful guide. Nevertheless all living things have much in common,in their chemical composition, their cellular structure, their laws ofgrowth, and their liability to injurious influences. We see this even inso trifling a fact as that the same poison often similarly affects plantsand animals; or that the poison secreted by the gall-fly produces monstrousgrowths on the wild rose or oak-tree. With all organic beings, exceptingperhaps some of the very lowest, sexual reproduction seems to beessentially similar. With all, as far as is at present known, the germinalvesicle is the same; so that all organisms start from a common origin. Ifwe look even to the two main divisions--namely, to the animal and vegetablekingdoms--certain low forms are so far intermediate in character thatnaturalists have disputed to which kingdom they should be referred. AsProfessor Asa Gray has remarked, "the spores and other reproductive bodiesof many of the lower algae may claim to have first a characteristicallyanimal, and then an unequivocally vegetable existence." Therefore, on theprinciple of natural selection with divergence of character, it does notseem incredible that, from some such low and intermediate form, bothanimals and plants may have been developed; and, if we admit this, we mustlikewise admit that all the organic beings which have ever lived on thisearth may be descended from some one primordial form. But this inferenceis chiefly grounded on analogy, and it is immaterial whether or not it beaccepted. No doubt it is possible, as Mr. G.H. Lewes has urged, that atthe first commencement of life many different forms were evolved; but ifso, we may conclude that only a very few have left modified descendants. For, as I have recently remarked in regard to the members of each greatkingdom, such as the Vertebrata, Articulata, etc., we have distinctevidence in their embryological, homologous, and rudimentary structures,that within each kingdom all the members are descended from a singleprogenitor.

When the views advanced by me in this volume, and by Mr. Wallace or whenanalogous views on the origin of species are generally admitted, we candimly foresee that there will be a considerable revolution in naturalhistory. Systematists will be able to pursue their labours as at present;but they will not be incessantly haunted by the shadowy doubt whether thisor that form be a true species. This, I feel sure and I speak afterexperience, will be no slight relief. The endless disputes whether or notsome fifty species of British brambles are good species will cease. Systematists will have only to decide (not that this will be easy) whetherany form be sufficiently constant and distinct from other forms, to becapable of definition; and if definable, whether the differences besufficiently important to deserve a specific name. This latter point willbecome a far more essential consideration than it is at present; fordifferences, however slight, between any two forms, if not blended byintermediate gradations, are looked at by most naturalists as sufficient toraise both forms to the rank of species.

Hereafter we shall be compelled to acknowledge that the only distinctionbetween species and well-marked varieties is, that the latter are known, orbelieved to be connected at the present day by intermediate gradations,whereas species were formerly thus connected. Hence, without rejecting theconsideration of the present existence of intermediate gradations betweenany two forms, we shall be led to weigh more carefully and to value higherthe actual amount of difference between them. It is quite possible thatforms now generally acknowledged to be merely varieties may hereafter bethought worthy of specific names; and in this case scientific and commonlanguage will come into accordance. In short, we shall have to treatspecies in the same manner as those naturalists treat genera, who admitthat genera are merely artificial combinations made for convenience. Thismay not be a cheering prospect; but we shall at least be freed from thevain search for the undiscovered and undiscoverable essence of the termspecies.

The other and more general departments of natural history will rise greatlyin interest. The terms used by naturalists, of affinity, relationship,community of type, paternity, morphology, adaptive characters, rudimentaryand aborted organs, etc., will cease to be metaphorical and will have aplain signification. When we no longer look at an organic being as asavage looks at a ship, as something wholly beyond his comprehension; whenwe regard every production of nature as one which has had a long history;when we contemplate every complex structure and instinct as the summing upof many contrivances, each useful to the possessor, in the same way as anygreat mechanical invention is the summing up of the labour, the experience,the reason, and even the blunders of numerous workmen; when we thus vieweach organic being, how far more interesting--I speak from experience--doesthe study of natural history become!

A grand and almost untrodden field of inquiry will be opened, on the causesand laws of variation, on correlation, on the effects of use and disuse, onthe direct action of external conditions, and so forth. The study ofdomestic productions will rise immensely in value. A new variety raised byman will be a far more important and interesting subject for study than onemore species added to the infinitude of already recorded species. Ourclassifications will come to be, as far as they can be so made,genealogies; and will then truly give what may be called the plan ofcreation. The rules for classifying will no doubt become simpler when wehave a definite object in view. We possess no pedigree or armorialbearings; and we have to discover and trace the many diverging lines ofdescent in our natural genealogies, by characters of any kind which havelong been inherited. Rudimentary organs will speak infallibly with respectto the nature of long-lost structures. Species and groups of species whichare called aberrant, and which may fancifully be called living fossils,will aid us in forming a picture of the ancient forms of life. Embryologywill often reveal to us the structure, in some degree obscured, of theprototypes of each great class.

When we can feel assured that all the individuals of the same species, andall the closely allied species of most genera, have, within a not veryremote period descended from one parent, and have migrated from some onebirth-place; and when we better know the many means of migration, then, bythe light which geology now throws, and will continue to throw, on formerchanges of climate and of the level of the land, we shall surely be enabledto trace in an admirable manner the former migrations of the inhabitants ofthe whole world. Even at present, by comparing the differences between theinhabitants of the sea on the opposite sides of a continent, and the natureof the various inhabitants of that continent in relation to their apparentmeans of immigration, some light can be thrown on ancient geography.

The noble science of geology loses glory from the extreme imperfection ofthe record. The crust of the earth, with its embedded remains, must not belooked at as a well-filled museum, but as a poor collection made at hazardand at rare intervals. The accumulation of each great fossiliferousformation will be recognised as having depended on an unusual occurrence offavourable circumstances, and the blank intervals between the successivestages as having been of vast duration. But we shall be able to gauge withsome security the duration of these intervals by a comparison of thepreceding and succeeding organic forms. We must be cautious in attemptingto correlate as strictly contemporaneous two formations, which do notinclude many identical species, by the general succession of the forms oflife. As species are produced and exterminated by slowly acting and stillexisting causes, and not by miraculous acts of creation; and as the mostimportant of all causes of organic change is one which is almostindependent of altered and perhaps suddenly altered physical conditions,namely, the mutual relation of organism to organism--the improvement of oneorganism entailing the improvement or the extermination of others; itfollows, that the amount of organic change in the fossils of consecutiveformations probably serves as a fair measure of the relative, though notactual lapse of time. A number of species, however, keeping in a bodymight remain for a long period unchanged, whilst within the same period,several of these species, by migrating into new countries and coming intocompetition with foreign associates, might become modified; so that we mustnot overrate the accuracy of organic change as a measure of time.

In the future I see open fields for far more important researches. Psychology will be securely based on the foundation already well laid byMr. Herbert Spencer, that of the necessary acquirement of each mental powerand capacity by gradation. Much light will be thrown on the origin of manand his history.

Authors of the highest eminence seem to be fully satisfied with the viewthat each species has been independently created. To my mind it accordsbetter with what we know of the laws impressed on matter by the Creator,that the production and extinction of the past and present inhabitants ofthe world should have been due to secondary causes, like those determiningthe birth and death of the individual. When I view all beings not asspecial creations, but as the lineal descendants of some few beings whichlived long before the first bed of the Cambrian system was deposited, theyseem to me to become ennobled. Judging from the past, we may safely inferthat not one living species will transmit its unaltered likeness to adistinct futurity. And of the species now living very few will transmitprogeny of any kind to a far distant futurity; for the manner in which allorganic beings are grouped, shows that the greater number of species ineach genus, and all the species in many genera, have left no descendants,but have become utterly extinct. We can so far take a prophetic glanceinto futurity as to foretell that it will be the common and widely spreadspecies, belonging to the larger and dominant groups within each class,which will ultimately prevail and procreate new and dominant species. Asall the living forms of life are the lineal descendants of those whichlived long before the Cambrian epoch, we may feel certain that the ordinarysuccession by generation has never once been broken, and that no cataclysmhas desolated the whole world. Hence, we may look with some confidence toa secure future of great length. And as natural selection works solely byand for the good of each being, all corporeal and mental endowments willtend to progress towards perfection.

It is interesting to contemplate a tangled bank, clothed with many plantsof many kinds, with birds singing on the bushes, with various insectsflitting about, and with worms crawling through the damp earth, and toreflect that these elaborately constructed forms, so different from eachother, and dependent upon each other in so complex a manner, have all beenproduced by laws acting around us. These laws, taken in the largest sense,being Growth with reproduction; Inheritance which is almost implied byreproduction; Variability from the indirect and direct action of theconditions of life, and from use and disuse; a Ratio of Increase so high asto lead to a Struggle for Life, and as a consequence to Natural Selection,entailing Divergence of Character and the Extinction of less improvedforms. Thus, from the war of nature, from famine and death, the mostexalted object which we are capable of conceiving, namely, the productionof the higher animals, directly follows. There is grandeur in this view oflife, with its several powers, having been originally breathed by theCreator into a few forms or into one; and that, whilst this planet has gonecircling on according to the fixed law of gravity, from so simple abeginning endless forms most beautiful and most wonderful have been, andare being evolved.