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Updated: May 14, 2025
What we have to consider and to investigate is whether the legs arose as a gametic mutation or as a direct result of locomotion on land. The general result of clinical and experimental evidence is to show that the hormone of the thyroid is necessary to normal development.
One specimen of Plaice of this type was kept alive in the aquarium, and it lay on its side, buried itself in the sand, and when disturbed swam horizontally, like a normal specimen. The abnormalities are undoubtedly mutations of gametic origin.
The question therefore is whether the somatic modification is inherited. The fact that Proteus does not rapidly become as deeply coloured when exposed to light as ordinary Amphibia shows that the gametic factors for pigmentation have been modified as well as the somatic tissues.
But the degree of development of a structure whose presence is due to gametic factors may very probably be modified by external stimulation, and the modification may become hereditary. If the views here advocated are true, the two processes mutation and modification must be always acting together and affecting the development not only of the individual but of any organ or structure.
Castration, then, tends to prove that in Birds the development of the male characters is not so closely dependent on the stimulation of testicular hormone as in Mammals. The characters must therefore be developed by heredity in the soma, which implies that the soma must itself be differentiated in the two sexes. The development must therefore be more in the nature of gametic coupling.
It is almost as though a man should discover that blindness in a certain case was due to a congenital, i.e. gametic, defect, and should then scoff at the idea that any person could become blind by disease. Some of those who specialise in the investigation of genetics seem to give inadequate consideration to other branches of biology.
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.
We have here, I think, an example of the essential difference between mutations and somatic modifications. Absence of the gametic factor or factors for pigmentation results in albinism, and no amount of exposure to light produces pigmentation in albinos, e.g. albino Axolotls which are well known in captivity. Absence of light, on the other hand, prevents the development of pigment.
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