It may be paradoxical, but it is strictly true, that the fact of particular species having remained unaltered during four thousand years, does not add the slightest weight to the evidence in favour of the fixity of Species. “What!” some may exclaim, “do you pretend that four thousand years is not a period long enough to prove the fixity of animal forms?” Yes; I affirm that four thousand, or forty thousand, prove no more than four. It is only by a fallacy that the opposite opinion could gain acceptance. You would not suppose that I had strengthened my case if, instead of contenting myself with stating reasons once, I repeated these same reasons during forty successive pages; you would remind me that this iteration was not cumulation, and that no force was given to my fortieth assertion which the first wanted. Why, then, do you ask me to accept the repetition of the same fact four thousand times over, as an increase of evidence? It is a familiar fact that like produces like, that dogs resemble dogs, and do not resemble buffaloes; this fact is, of course, deepened in our conviction by the unvarying evidence we see around us, and is guaranteed by the philosophical axiom that like causes produce like effects; but when once such a conception is formed, it can gain no fresh strength from any particular instance. If we believe that crows are black, we do not hold that belief more firmly when we are shown that crows were black four thousand years ago. In like manner, if it is an admitted fact that individuals always reproduce individuals closely resembling themselves, it is not a whit more surprising that the dogs of Victoria should resemble the dogs of Semiramis, than that they should resemble their parents: the chain of four thousand years is made up of many links, each link being a repetition of the other. So long as a single pair of dogs resembling each other unite, so long will there be specimens of that species; simply because the children inherit the characteristics of the parents. So long as Negroes marry with Negroes, and Jews with Jews, so long must there be a perpetuation of the Negro and Jewish types; but the tenth generation adds nothing to the evidence of the first, nor the ten-thousandth to the tenth.
I believe that this fallacy, which destroys the whole value of the Cuvierian argument, has not before been pointed out; and even now, you may, perhaps, ask if the fixity of Species is not proved by the fact that like produces like? So far from this, that it is only by the aid of such a fact in organic nature that we can imagine new species to have arisen: in other words, those who believe in the variability of Species, and the introduction of new forms by means of modification from the old, always invoke the law of hereditary transmission as the means of establishing accidental variations. Thus, let us suppose the Egyptian king to have had one hundred dogs, all of them staghounds, and no other form of dog to have existed at that time in that country; the dog species would be represented by the staghound. These staghounds would transmit to their offspring all their specific characters. But, as every one knows, however much dogs may resemble each other, they always present individual differences in size, colour, strength, intelligence, &c. Now, if any one of these differences should happen to become marked, and to increase by the intermarriage of two dogs similarly distinguished by the marked peculiarity, this peculiarity would in time become established by hereditary transmission, and would form the starting-point of a new race of dogs—say the greyhound—unless it were obliterated by intermarriage with dogs of the old type. In the former case, we should have two races of dogs among the descendants of those figured on the Egyptian tombs; but as one of these races would still preserve the original staghound type, Cuvier would refer to it as a proof that species had not varied. We, on the other hand, should point to the greyhound as proof that animal forms are variable, and that a new form had arisen from modification of the old.
An objection will at once be raised to this illustration, to the effect that all zoologists admit the possibility of new Varieties, or Races, being formed; but they deny that new Species can be formed. It is here that the equivoque of the word Species prevents a clear understanding of each other’s argument. Whiteness may justly be said to be unalterable; but white things may vary—they may become gray, or yellow. In like manner Species must be invariable, because Species is a word indicating a particular group of characters; but animals may vary in these characters: they may present some of the characters less, or more, developed; and they may even want some of them. Now as there is no absolute standard of what constitutes Species, what Sub-species, and what Varieties, it becomes impossible to say whether any individual variation in an animal form shall constitute a new Variety, or a new Species. With regard to dogs the differences between the various races are so numerous, and so marked, as would suffice to constitute species and even genera, in other groups of animals.
We must relinquish the idea of proving anything by the paintings and sculptures of the ancients. When we find an Egyptian plough closely resembling the plough still in use in some places, we may identify it as of the same “Species” as our own; but this does not disprove the fact that steam-ploughs, and ploughs of various construction, have been since invented, all of them being modifications of the original type. Formerly, and for many years, the stage-coach was our approved mode of conveyance—and it is still kept up in some districts; nevertheless, modifications of coach-road into tramroad, and tramroad into railroad, have gradually resulted in a mode of conveyance utterly unlike the stage-coach. It is the same with animals.
Let us never forget that Species have no existence. Only individuals exist, and these all vary more or less from each other. When the variations are slight, they have no name; when they are more marked, and are transmitted from one generation to another, they constitute particular Faces, or Varieties; when the differences are still more marked they constitute Sub-species; but, as Mr. Darwin says, “Certainly no clear line of demarcation has yet been drawn between Species and Sub-species; that is, the forms which in the opinion of some naturalists come very near to, but do not quite arrive at the rank of Species; or again, between Sub-species and well-marked Varieties, or between lesser Varieties and individual differences. These differences blend into each other in an insensible series; and a series impresses the mind with the idea of an actual passage.” But the same process of divergence which establishes Varieties out of individual differences, and Species out of Varieties, also serves to establish Genera out of Species, Orders out of Genera, and Classes out of Orders. It is, doubtless, difficult to conceive by what process of modification, two animals of distinct Genera, say a dog and a cat, were produced from a common stock; but organic analogies in abundance render it easy of belief. If we knew as much of zoology as we do of embryology, in respect of the affinities of divergent forms, it would be far less surprising that two different Genera should arise from a common stock, than that all the various parts of the skeleton should arise from a common osseous element. We know that the jaws are identical with arms and legs—both being divergent modifications of a common osseous structure. We know that the arm of a man is identical with the fin of a whale, or the wing of a bird. The differences here in form, size, and function are much greater than the differences which establish orders and classes in the animal series. Unless animal forms were modifications of some common type, it would be difficult to explain their remarkable affinities. As Mr. Darwin says, “It is a truly wonderful fact—the wonder of which we are apt to overlook from familiarity—that all animals and all plants throughout all time and space should be related to each other in group, subordinate to group, in the manner which we everywhere behold, namely, varieties of the same species most closely related together, species of the same genus less closely and unequally related together, forming sections and sub-genera, species of distinct genera much less closely related, and genera related in different degrees, forming sub-families, families, orders, sub-classes, and classes. The several subordinate groups in any class cannot be ranked in a single file, but seem rather to be clustered round points, and these round other points, and so on in almost endless circles. On the view that each species has been independently created, I can see no explanation of this great fact in the classification of all organic beings; but to the best of my judgment it is explained through inheritance, and the complex action of natural selection, entailing extinction and divergence of character. The affinities of all the beings of the same class have sometimes been represented by a great tree. I believe this simile largely speaks the truth. The green and budding twigs may represent existing species; and those produced during each former year may represent the long succession of extinct species. At each period of growth all the growing twigs have tried to branch out on all sides, and to overtop and kill the surrounding twigs and branches, in the same manner as species and groups of species have tried to overmaster other species in the great struggle for life. The limbs divided into great branches, and these into lesser branches, were themselves once, when the tree was small, budding twigs; and this connection of the former and present buds by ramifying branches, may well represent the classification of all extinct and living species in groups subordinate to groups. Of the many twigs which flourished when the tree was a mere bush, only two or three, now grown into great branches, yet survive and bear all the other branches. So with the species which lived during long-past geological periods, very few now have living and modified descendants.... As buds give rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler branch: so by generation, I believe, it has been with the great Tree of Life, which fills with its dead and broken branches the crust of the earth, and covers the surface with its ever-branching and beautiful ramifications.”[14]
It will not be expected that in these brief and desultory remarks I should touch on all, or nearly all, the important points in the discussion respecting the Fixity of Species. Mr. Darwin’s book is in everybody’s hands, and my object has been to facilitate, if possible, the comprehension of his book, and the adoption of a more philosophical hypothesis, by pointing out the weakness of the chief argument on the other side. There is one more argument which may be noticed—the more so as it is constantly adduced with triumph by the one school, and admitted as a difficulty by the other. Its force is so great that it prevents many from accepting the development hypothesis. It is the argument founded on the alleged impossibility of Hybrids continuing the race. More than two or three generations of Hybrids, it is said, can never be maintained; after that, the new form perishes: thus clearly showing how Nature repudiates such amalgamations, and keeps her species jealously distinct and invariable. This argument is held to be the touchstone of the doctrine of species. I wish it were so; because, in that case, the question would no longer be one of hypothesis, since we have now the indubitable proof that some Hybrids are fertile unto the thirteenth generation and onwards.
A history of the various attempts which have been made to prove and disprove the fertility of Hybrids, would lead us beyond our limits; the curious reader is referred to the works cited below.[15] One decisive case alone shall be given here, and no one will dispute that it is decisive.
The hare (lepus timidus) is assuredly of a distinct species from the rabbit (lepus cuniculus). So distinct are these species, that any classification which should range them as one, would violate every accepted principle. The hare is solitary, the rabbit gregarious; the hare lives on the surface of the earth, the rabbit burrows under the surface; the hare makes her home among the bushes, the rabbit makes a sort of nest for her young in her burrow—keeping them there till they are weaned; the hare has reddish-brown flesh, the rabbit white flesh; while the odour exhaled by each, and the flavour of each, are unmistakeably different. The hare has many anatomical characters differing from those of the rabbit: such as greater length and strength of the hind legs, larger body, shorter intestine, thicker skin, firmer hair, and different colour. The hare breeds only twice or thrice a year, and at each litter has only two or four; the rabbit will breed eight times a year, and each time has four, six, seven, and even eight young ones. Finally, the two are violent foes: the rabbits always destroy the hares, and all sportsmen are aware that if the rabbits be suffered to multiply on an estate, there will be small chance of hares.
Nevertheless, between species so distinct as these, a new hybrid race has been reared by M. Rouy, of Angoulême, who each year sends to market upwards of a thousand of his Leporides, as he calls them. His object was primarily commercial, not scientific. His experiments, extending from 1847 to the present time, have not only been of great commercial value—introducing a new and valuable breed—but have excited the attention of scientific men, who are now availing themselves of his skill and experience to help them in the solution of minor problems. It is enough to note here, that these hybrids of the hare and the rabbit are fertile, not only with either hares or rabbits, but with each other. Thirteen generations have already been enumerated, and the last remains so vigorous that no cessation whatever is to be anticipated.