I can quite well understand, however, that many naturalists, and especially palæontologists, find it difficult to accept this conclusion. If we think only of those parts that actively function, and thus change by reason of their function, being strengthened by use and weakened and diminished in size by disuse, and if, further, we follow these parts through the history of whole geological epochs, we may certainly get the impression that the exercise of the parts has directly caused their phyletic evolution. The direction prescribed by utility in the course of the individual life and in the phylogeny is the same, and the intra-selection, that is, the selection of tissues within the individual animal, leads towards the same improvements as the selection of 'persons.' Thus it appears as if the phyletic variations followed those of the individual life, while in reality the reverse is true; the changes arising from variations in the germ are primary, and they determine the course of phylogenesis, while the tissue-selection in the individual life only elaborates and improves, according to the demands made upon it, the material afforded by the primordial equipment of the germ.
The American palæontologist, Osborn, cites the case of the horse's feet as an example in support of his view that modification brought about by use in the individual life must be transmitted in order that the phyletic transformations may be brought about, but this example is perhaps the best that could be chosen to prove the contrary. He supposes that, in every young horse, the means of locomotion are improved at every step, so to speak, through the contact with the ground, and I am quite willing to admit that this is so. But that only proves that, even now, an elaboration and improvement of the equipment which the germ affords is indispensable, as it has been at all times and in all animals, and thus that, notwithstanding the enormous number of generations which our modern horse has behind it, the functional acquirements of the individual have not yet been impressed upon the germ. Why not? Because the horse becomes perfect without this, and there was no reason why personal selection should perfect the primary constituents of the germ still further, since the finishing touch of perfection through use is readily afforded by the conditions of each individual life.
Moreover, when Osborn, Cope, and other palæontologists emphasize that, in phyletic evolutionary series, definite paths of evolutionary change are adhered to, and are not deviated from either to right or to left, they are undoubtedly right, but the conclusion which they draw is not justifiable, whether they assume with Nägeli that there is a power of development, a principle of perfecting, or whether, as Osborn does, they assume the transmission of the modifications brought about through use in the individual life. There remains a third possibility, that the quiet and constant evolution in a definite direction is guided by selection, and as, in passively useful parts, that principle alone is admissible, I see no justification for assuming it to be inoperative in regard to those which are actively functional. All these variations which have led up, for instance, to the modern form of the horse's foot are useful; if they were not, they could not have been produced either by use or by disuse in the individual life.
At the same time, here again, we are justified in inquiring whether the assumption of 'chance' germinal variations, which we have hitherto made with Darwin and Wallace, affords a sufficient basis for selection. Osborn says very neatly in this connexion, 'We see with Weismann and Galton the element of chance; but the dice appear to be loaded, and in the long run turn "sixes" up. Here arises the question: What loads the dice?'
Until recently we might have answered, 'external conditions'; it is they that load the dice one-sidedly, and condition that the same straight path of phylogenesis is adhered to, and exactly the same direction of variations is preferred and maintained. It has to be asked, however, whether this answer, which is certainly not absolutely incorrect, is sufficient by itself, whether the dice are not falsified and one-sidedly loaded in another sense, so that they always throw a preponderating number of the useful variations. We shall attempt very soon to solve this problem, but in the meantime I must refer to another argument in favour of assuming the Lamarckian principle, perhaps the most important and it may be thought the most difficult of all to refute, the so-called co-adaptation of the parts of an organism, that is, the fitting together of many individual organs for a common purposeful functioning.
LECTURE XXIV
OBJECTIONS TO THE THESIS THAT FUNCTIONAL
MODIFICATIONS ARE NOT TRANSMITTED
Giant stag as an example of co-adaptation or 'harmonious adaptation'—This occurs even in passively functioning parts—Skeleton of Arthropods—Stridulating organ of ants and crickets—Limbs of the mole-cricket—Wing-venation—Colorations which form mimetic pictures—Harmonious adaptations in worker-bees and ants—Degeneration of their wings and ovaries—The quality of food acts as a liberating stimulus—Vom Rath's case of drones fed with royal food—Transition-forms between females and workers—Wasmann's explanation of these—The Amazon ants—Two kinds of workers—Appendix: Zehnder on the case of ants—On the skeleton of Arthropods—Hering's interpretation of Ehrlich's Ricin experiments—Hering's position in regard to the transmission of functional modifications.