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As one of the most remarkable examples of metamorphosis and recapitulation in connexion with adaptation we will consider once more the case of the Flat-fishes which I have already mentioned in an earlier chapter. These fishes offer perhaps the best example of the difference between gametogenic mutations and adaptive modifications. Trans.
Finally it may be urged, although the question has not been directly discussed in this volume, that no biologist is justified in the present state of knowledge in dogmatically teaching the lay public that gametogenic characters are alone worthy of attention in questions of eugenics and sociology.
The general conclusions which are logically to be drawn from our present knowledge with regard to the problems of heredity and evolution in animals are in my opinion as follows: All attempts to explain adaptation by gametogenic mutations, or changes in gametic factors or 'genes, have completely failed, as Bateson himself has admitted.
The author's view is that are two kinds of variation in evolution, one somatogenic and due to external stimuli, acting either directly on passive tissues or indirectly through function, and the other gametogenic and due to changes in the chromosomes of the gametes which are spontaneous and not in any way due to modifications of the soma.
Theory supposes that somatic modifications by means of hormones affect the determinants in the gametes. But it is obvious that the black and white of Leghorn fowls and of guinea-pigs are gametogenic characters, and are strongly established in the gametes of their respective varieties.
For the present, therefore, we must conclude that feathers are not an adaptation, and not due to somatogenic modification, but must be result of a gametogenic mutation. Feathers, having been evolved, served in the wings and tail as important organs of flight.
No cases of mutation or gametogenic variation hitherto described exhibit a similar metamorphosis or recapitulation. Secondary sexual characters, usually in the male sex, correspond in their development with the development of maturity and functional activity in the gonads, and it has been proved that the latter influence the former by means of 'hormones' or internal secretions.
It might be agreed that if the determinants are stimulated or deprived of stimulation, the effect of the change should logically show itself from the beginning of development, and that therefore the process of metamorphosis or indirect development does not support the hormone theory any more than the theory of gametogenic mutations. This objection may be answered in the following way.
Since many breeds have upright ears, we cannot say that disuse of the external ear has produced lop-ears in domesticated rabbits generally, but in lop-eared breeds the ears are much enlarged; and though this may be gametogenic, the increased weight may have been the cause of the loss of the power to erect the ears.
Adaptations are due to somatogenic modifications, non-adaptive diagnostic characters to gametogenic mutations. It is a mistake to attempt to explain all the results of evolution by a principle.
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