The answer made to this argument was that coördinating parts vary together. In reply to which Spencer points to the following cases, which show that this is not so: The blind crayfish in the Kentucky caves have lost their eyes, but not the stalks that carry them. Again, the normal relation between the length of tongue and of beak in some varieties of pigeons is lost. The greater decrease in the jaws in some species of pet dogs than of the number of their teeth has caused the teeth to become crowded.[[18]] “I then argued that if coöperative parts, small in number, and so closely associated as these are, do not vary together, it is unwarrantable to allege that coöperative parts, which are very numerous and remote from one another, vary together.” Spencer puts himself here into the position of seriously maintaining that, because some coöperative parts do not vary together, therefore no coöperative parts have ever done so, and he has taken this position in the face of some well-known cases in which certain parts have been found to vary together.
[18]. It is curious that Spencer does not see that this case is as much against his point as in favor of it, since the unused teeth did not also degenerate.
In this same connection Spencer brings up the familiar pièce de résistance of the Lamarckian school, the giraffe. He recognizes that the chief traits in the structure of this animal are the result of natural selection, since its efforts to reach higher branches could not be the cause of the lengthening of the legs. But “the coadaptation of the parts, required to make the giraffe’s structure useful, is much greater than at first appears.” For example, the bones and the muscles of the hind-legs have been also altered, and Spencer argues that it is “impossible to believe” that all parts of the hind-quarters could have been coadapted to one another, and to all parts of the fore-quarters. A lack of coadaptation of a single muscle “would cause fatal results when high speed had to be maintained while escaping from an enemy.”
Spencer claims that, since 1886, when he first published this argument, nothing like an adequate response has been made; and I think he might have added that an adequate answer is not likely to be forthcoming, since nothing short of a demonstration of how the giraffe really evolved is likely to be considered as sufficient. Wallace’s reply, that the changes in question could have been brought about by natural selection, since similar changes have been brought about by artificial selection, is regarded as inadequate by Spencer, since it assumes a parallel which does not exist. Nevertheless, Wallace’s reply contains, in my opinion, the kernel of the explanation, in so far as it assumes that congenital variation[[19]] may suffice to account for the origin of a form even as bizarre as that of the giraffe. The ancon ram and the turnspit dog were marked departures from the normal types, and yet their parts were sufficiently coördinated for them to carry out the usual modes of progression. It would not have been difficult, if we adopted Spencer’s mode of arguing, to show that these new forms could not possibly have arisen as the result of congenital variations.
[19]. Wallace assumes fluctuating variation to suffice, but in this I cannot agree with him.
Again, it might be argued that the large, powerful dray-horse could not have arisen through a series of variations from the ordinary horse, because, even if variations in the right direction occurred in the fore-quarters, it is unlikely that similar variations would occur in the hind-quarters, etc. Yet the feat has been accomplished, and while it is difficult to prove that the inheritance of acquired characters has not had a hand in the process, it is improbable that this has been the case, but rather that artificial selection of some kind of variations has been the factor at work.
So long as the Lamarckian theory is supported by arguments like these, it can never hope to be established with anything more than a certain degree of probability. If it is correct, then its demonstration must come from experiment. This brings us to a consideration of the experimental evidence which has been supposed by some writers to give conclusive proof of the validity of the theory.
The best direct evidence in favor of the Lamarckian argument is that furnished by the experiments of Brown-Séquard. He found, as the result of injury to the nervous system of guinea-pigs, that epilepsy appeared in the adult animal, and that young born from these epileptic parents became also epileptic. Still more important was his discovery that, after an operation on the nerves, as a result of which certain organs, the ear or the leg, for instance, are affected, the same affection appears in the young born from such parents. These results of Brown-Séquard have been vouched for by two of his assistants, and his results in regard to the inheritance of epilepsy have been confirmed by Obersteiner, and by Luciani on dogs. Equally important is their later confirmation, as far as the main facts go, by Romanes.
Brown-Séquard gives the following summary of his results. I follow Romanes’ translation in his book on “Darwin and after Darwin,” where there is also given a careful analysis of Brown-Séquard’s results, as well as the outcome of the experiments of Romanes himself. The summary is as follows:—
1. “Appearance of epilepsy in animals born of parents which had been rendered epileptic by an injury to the spinal cord.