II.—Succession of Species, Broken or Linked?
To the hypothesis that existing are modifications of extinct species, Cuvier replied that traces of modification were due from the fossil world. "You ought," he said, "to be able to show the intermediate forms between the palæotherium and existing hoofed quadrupeds."
The progress of palæontology since 1830 has brought to light many missing links unknown to the founder of the science. The discovery of the remains of the hipparion supplied one of the links required by Cuvier, and it is significant that the remains of such three-toed horses are found only in deposits of that tertiary period which intervene between the older palæotherian one and the newer strata in which the modern horse first appears to have lost its lateral hooflets.
The molar series of the horse includes six large complex grinders individually recognisable by developmental characters. The representative of the first premolar is minute and soon shod. Its homologue in palæotherium is functionally developed and retained, that type-dentition being adhered to. In hipparion this tooth is smaller than in palæotherium, but functional and permanent. The transitory and singularly small and simple denticle in the horse exemplifies the rudiment of an ancestral structure in the same degree as do the hoofless splint-bones; just as the spurious hoofs dangling therefrom in hipparion are retained rudiments of the functionally developed lateral hoofs in the broader foot of palæotherium.
Other missing links of this series of species have also been supplied.
How then is the origin of these intermediate gradations to be interpreted? If the alternative—species by miracle or by law—be applied to palæotherium, paloplotherium, anchitherium, hipparion, equus, I accept the latter without misgiving, and recognise such law as continuously operative throughout tertiary time.
In respect to its law of operation we may suppose Lamarck to say, "as the surface of the earth consolidated, the larger and more produced mid-hoof of the old three-toed pachyderius took a greater share in sustaining the animal's weight; and more blood being required to meet the greater demand of the more active mid-toe, it grew; whilst, the side-toes, losing their share of nourishment and becoming more and more withdrawn from use, shrank"—and so on. Mr. Darwin, I conceive, would modify this by saying that some individuals of palæotherium happening to be born with a larger and longer middle toe, and with shorter and smaller side-toes, such variety was better adapted to prevailing altered conditions of the earth's surface than the parental form; and so on, until finally the extreme equine modifications of foot came to be "naturally selected." But the hypothesis of appetency and volition, as of natural selection, are less applicable, less intelligible, in connection with the changes in the teeth.
I must further observe that to say the palæotherium has graduated into equus by "natural selection" is an explanation of the process of the same kind and value as that by which the secretion of bile was attributed to the "appetency" of the liver for the elements of bile. One's surprise is that such explanatory devices should not have died out with the "archeus faber," the "nisus formations," and other self-deceiving, world-beguiling simulacra of science, with the last century; and that a resuscitation should have had any success in the present.
What, then, are the facts on which any reasonable or intelligible conception can be formed of the mode of operation of the derivative law exemplified in the series linking on palæotherium to equus? A very significant one is the following. A modern horse occasionally comes into the world with the supplementary ancestral hoofs. From Valerius Maximus, who attributes the variety to Bucephalus downwards, such "polydactyle" horses have been noted as monsters and marvels. In one of the latest examples, the inner splint-bone, answering to the second metacarpal of the pentadactyle foot, supported phalanges and a terminal hoof resembling the corresponding one in hipparion. And the pairing of horses with the meterpodials bearing, according to type, phalanges and hoofs might restore the race of hipparions.
Now, the fact suggesting such possibility teaches that the change would be sudden and considerable; it opposes the idea that species are transmuted by minute and slow degrees. It also shows that a species might originate independently of the operation of any external influence; that change of structure would precede that of use and habit; that appetency, impulse, ambient medium, fortuitous fitness of surrounding circumstances, or a personified "selecting nature" would have had no share in the transmutative act.