though sometimes applied to uses not normal,--which cannot be considered as mere representative parts, for they are sometimes capable of performing their proper function,--which are always best developed, and sometimes only developed, during a very early period of life,--and which are of admitted high importance in cla.s.sification,--were shown to be simply explicable on our theory of common descent._Why do we wish to reject the theory of common descent?_
Thus have many general facts, or laws, been included under one explanation; and the difficulties encountered are those which would naturally result from our acknowledged ignorance. And why should we not admit this theory of descent{514}? Can it be shown that organic beings in a natural state are _all absolutely invariable_? Can it be said that the _limit of variation_ or the number of varieties capable of being formed under domestication are known? Can any distinct line be drawn _between a race and a species_? To these three questions we may certainly answer in the negative. As long as species were thought to be divided and defined by an impa.s.sable barrier of _sterility_, whilst we were ignorant of geology, and imagined that the _world was of short duration_, and the number of its past inhabitants few, we were justified in a.s.suming individual creations, or in saying with Whewell that the beginnings of all things are hidden from man. Why then do we feel so strong an inclination to reject this theory--especially when the actual case of any two species, or even of any two races, is adduced--and one is asked, have these two originally descended from the same parent womb?
I believe it is because we are always slow in admitting any great change of which we do not see the intermediate steps. The mind cannot grasp the full meaning of the term of a million or hundred million years, and cannot consequently add up and perceive the full effects of small successive variations acc.u.mulated during almost infinitely many generations. The difficulty is the same with that which, with most geologists, it has taken long years to remove, as when Lyell propounded that great valleys{515} were hollowed out [and long lines of inland cliffs had been formed] by the slow action of the waves of the sea. A man may long view a grand precipice without actually believing, though he may not deny it, that thousands of feet in thickness of solid rock once extended over many square miles where the open sea now rolls; without fully believing that the same sea which he sees beating the rock at his feet has been the sole removing power.
{514} This question forms the subject of what is practically a section of the final chapter of the _Origin_ (Ed. i. p. 480, vi. p.
657).
{515} _Origin_, Ed. i. p. 481, vi. p. 659.
Shall we then allow that the three distinct species of rhinoceros{516} which separately inhabit Java and Sumatra and the neighbouring mainland of Malacca were created, male and female, out of the inorganic materials of these countries? Without any adequate cause, as far as our reason serves, shall we say that they were merely, from living near each other, created very like each other, so as to form a section of the genus dissimilar from the African section, some of the species of which section inhabit very similar and some very dissimilar stations? Shall we say that without any apparent cause they were created on the same generic type with the ancient woolly rhinoceros of Siberia and of the other species which formerly inhabited the same main division of the world: that they were created, less and less closely related, but still with interbranching affinities, with all the other living and extinct mammalia? That without any apparent adequate cause their short necks should contain the same number of vertebrae with the giraffe; that their thick legs should be built on the same plan with those of the antelope, of the mouse, of the hand of the monkey, of the wing of the bat, and of the fin of the porpoise. That in each of these species the second bone of their leg should show clear traces of two bones having been soldered and united into one; that the complicated bones of their head should become intelligible on the supposition of their having been formed of three expanded vertebrae; that in the jaws of each when dissected young there should exist small teeth which never come to the surface. That in possessing these useless abortive teeth, and in other characters, these three rhinoceroses in their embryonic state should much more closely resemble other mammalia than they do when mature. And lastly, that in a still earlier period of life, their arteries should run and branch as in a fish, to carry the blood to gills which do not exist. Now these three species of rhinoceros closely resemble each other; more closely than many generally acknowledged races of our domestic animals; these three species if domesticated would almost certainly vary, and races adapted to different ends might be selected out of such variations. In this state they would probably breed together, and their offspring would possibly be quite, and probably in some degree, fertile; and in either case, by continued crossing, one of these specific forms might be absorbed and lost in another. I repeat, shall we then say that a pair, or a gravid female, of each of these three species of rhinoceros, were separately created with deceptive appearances of true relationship, with the stamp of inutility on some parts, and of conversion in other parts, out of the inorganic elements of Java, Sumatra and Malacca? or have they descended, like our domestic races, from the same parent-stock? For my own part I could no more admit the former proposition than I could admit that the planets move in their courses, and that a stone falls to the ground, not through the intervention of the secondary and appointed law of gravity, but from the direct volition of the Creator.
{516} The discussion on the three species of _Rhinoceros_ which also occurs in the Essay of 1842, p. 48, was omitted in Ch. XIV of the _Origin_, Ed. i.
Before concluding it will be well to show, although this has incidentally appeared, how far the theory of common descent can legitimately be extended{517}. If we once admit that two true species of the same genus can have descended from the same parent, it will not be possible to deny that two species of two genera may also have descended from a common stock. For in some families the genera approach almost as closely as species of the same genus; and in some orders, for instance in the monocotyledonous plants, the families run closely into each other. We do not hesitate to a.s.sign a common origin to dogs or cabbages, because they are divided into groups a.n.a.logous to the groups in nature.
Many naturalists indeed admit that all groups are artificial; and that they depend entirely on the extinction of intermediate species. Some naturalists, however, affirm that though driven from considering sterility as the characteristic of species, that an entire incapacity to propagate together is the best evidence of the existence of natural genera. Even if we put on one side the undoubted fact that some species of the same genus will not breed together, we cannot possibly admit the above rule, seeing that the grouse and pheasant (considered by some good ornithologists as forming two families), the bull-finch and canary-bird have bred together.
{517} This corresponds to a paragraph in the _Origin_, Ed. i. p.
483, vi. p. 662, where it is a.s.sumed that animals have descended "from at most only four or five progenitors, and plants from an equal or lesser number." In the _Origin_, however, the author goes on, Ed. i. p. 484, vi. p. 663: "a.n.a.logy would lead me one step further, namely, to the belief that all animals and plants have descended from some one prototype."
No doubt the more remote two species are from each other, the weaker the arguments become in favour of their common descent. In species of two distinct families the a.n.a.logy, from the variation of domestic organisms and from the manner of their intermarrying, fails; and the arguments from their geographical distribution quite or almost quite fails. But if we once admit the general principles of this work, as far as a clear unity of type can be made out in groups of species, adapted to play diversified parts in the economy of nature, whether shown in the structure of the embryonic or mature being, and especially if shown by a community of abortive parts, we are legitimately led to admit their community of descent. Naturalists dispute how widely this unity of type extends: most, however, admit that the vertebrata are built on one type; the articulata on another; the mollusca on a third; and the radiata on probably more than one. Plants also appear to fall under three or four great types. On this theory, therefore, all the organisms _yet discovered_ are descendants of probably less than ten parent-forms.
_Conclusion._
My reasons have now been a.s.signed for believing that specific forms are not immutable creations{518}. The terms used by naturalists of affinity, unity of type, adaptive characters, the metamorphosis and abortion of organs, cease to be metaphorical expressions and become intelligible facts. We no longer look at an organic being as a savage does at a ship{519} or other great work of art, as at a thing wholly beyond his comprehension, but as a production that has a history which we may search into. How interesting do all instincts become when we speculate on their origin as hereditary habits, or as slight congenital modifications of former instincts perpetuated by the individuals so characterised having been preserved. When we look at every complex instinct and mechanism as the summing up of a long history of contrivances, each most useful to its possessor, nearly in the same way as when we look at a great mechanical invention as the summing up of the labour, the experience, the reason, and even the blunders of numerous workmen. How interesting does the geographical distribution of all organic beings, past and present, become as throwing light on the ancient geography of the world. Geology loses glory{520} from the imperfection of its archives, but it gains in the immensity of its subject. There is much grandeur in looking at every existing organic being either as the lineal successor of some form now buried under thousands of feet of solid rock, or as being the co-descendant of that buried form of some more ancient and utterly lost inhabitant of this world. It accords with what we know of the laws impressed by the Creator{521} on matter that the production and extinction of forms should, like the birth and death of individuals, be the result of secondary means. It is derogatory that the Creator of countless Universes should have made by individual acts of His will the myriads of creeping parasites and worms, which since the earliest dawn of life have swarmed over the land and in the depths of the ocean. We cease to be astonished{522} that a group of animals should have been formed to lay their eggs in the bowels and flesh of other sensitive beings; that some animals should live by and even delight in cruelty; that animals should be led away by false instincts; that annually there should be an incalculable waste of the pollen, eggs and immature beings; for we see in all this the inevitable consequences of one great law, of the multiplication of organic beings not created immutable. From death, famine, and the struggle for existence, we see that the most exalted end which we are capable of conceiving, namely, the creation of the higher animals{523}, has directly proceeded. Doubtless, our first impression is to disbelieve that any secondary law could produce infinitely numerous organic beings, each characterised by the most exquisite workmanship and widely extended adaptations: it at first accords better with our faculties to suppose that each required the fiat of a Creator.
There{524} is a [simple] grandeur in this view of life with its several powers of growth, reproduction and of sensation, having been originally breathed into matter under a few forms, perhaps into only one{525}, and that whilst this planet has gone cycling onwards according to the fixed laws of gravity and whilst land and water have gone on replacing each other--that from so simple an origin, through the selection of infinitesimal varieties, endless forms most beautiful and most wonderful have been evolved.
{518} This sentence corresponds, not to the final section of the _Origin_, Ed. i. p. 484, vi. p. 664, but rather to the opening words of the section already referred to (_Origin_, Ed. i. p. 480, vi. p. 657).
{519} This simile occurs in the Essay of 1842, p. 50, and in the _Origin_, Ed. i. p. 485, vi. p. 665, _i.e._ in the final section of Ch. XIV (vi. Ch. XV). In the MS. there is some erasure in pencil of which I have taken no notice.
{520} An almost identical sentence occurs in the _Origin_, Ed. i.
p. 487, vi. p. 667. The fine prophecy (in the _Origin_, Ed. i. p.
486, vi. p. 666) on "the almost untrodden field of inquiry" is wanting in the present Essay.
{521} See the last paragraph on p. 488 of the _Origin_, Ed. i., vi.
p. 668.
{522} A pa.s.sage corresponding to this occurs in the sketch of 1842, p. 51, but not in the last chapter of the _Origin_.
{523} This sentence occurs in an almost identical form in the _Origin_, Ed. i. p. 490, vi. p. 669. It will be noted that man is not named though clearly referred to. Elsewhere (_Origin_, Ed. i.
p. 488) the author is bolder and writes "Light will be thrown on the origin of man and his history." In Ed. vi. p. 668, he writes "Much light &c."
{524} For the history of this sentence (with which the _Origin of Species_ closes) see the Essay of 1842, p. 52, note 2{Note 184}: also the concluding pages of the Introduction.
{525} These four words are added in pencil between the lines.