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At the same time there are some somatic sex-characters, e.g. in insects and birds, which do not appear to be correlated with changes in the gonads, and which are probably gametogenic, not somatogenic in origin. The theory of the heredity of somatogenic modifications is not in opposition to the mutation theory.
The majority of evolutionists in recent years have taught that influences exerted through the soma have no effect on the determinants in the chromosomes of the gametes, that all hereditary variations are gametogenic and none somatogenic.
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.
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.
But experiments of this kind are from the nature of the case difficult if not impossible. There is, however, another method namely, to take a character which is certainly to some extent hereditary, and then to ascertain by experiment if it is 'acquired. If it be proved that a hereditary character was originally somatogenic, it follows that somatogenic characters in time become hereditary. Zool.
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.
When the same male was mated with a normal black female the offspring were black with red hairs interspersed. All these experiments are open to the following criticism. It has been the main argument of this volume that there are two distinct kinds of characters in all organisms namely, those of somatogenic origin and those of gametogenic origin.
The real problem, then, is the sex-limited heredity, and we shall consider later whether in this kind of heredity also there are characters of internal as well as external origin, blastogenic as well as somatogenic. Mendelism And The Heredity Of Sex
Satisfactory evidence by direct experiment of the inheritance of somatogenic modifications due to external stimuli cannot be said to have been yet produced, and, as I have shown, such evidence from the nature of the case must be very difficult to obtain.
In endeavouring to answer this question there are only two alternatives: either the characters are blastogenic that is, they arise from some change in the gametocytes occurring somewhere in the succession of cell-divisions of these cells or they arise in the soma and are impressed on the gametocytes by the influence of the soma within which these gametocytes are contained that is to say, they are somatogenic.
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