as a mutation in his cultures of _Drosphilia_. The character was eosin colour in the eye instead of the red colour of the eye in the original fly. In the female this was dark eosin colour, in the male yellowish eosin. But this case differs from the characters particularly under consideration here in two points: (1) there is no suggestion that it was adaptive, (2) or that it was influenced by hormones from the gonads. No character whose development is dependent in greater or less degree on the stimulation of some substance derived from the gonads can have originated as a mutation, because the term mutation means a new character which develops in the soma as a result of the loss or gain of some factor or determinant in the chromosomes. To say that certain mutations consist of new factors which only the development of characters in the soma when the part of the soma concerned is stimulated by a hormone, is a mere assertion unsupported at present by any evidence. As an example of the way in which Mendelians misunderstand the problem to be considered, I may refer to Doncaster's book, _The Determination of Sex_ [Footnote: Camb. Univ. 1914, p. 99.] in which he remarks: 'It follows that the secondary sexual characters cannot arise simply from the action of hormones; they must be due to differences in the tissues of the body, and the activity of the ovary or testis must be regarded rather as a stimulus to their development than as their source of origin.' This seems to imply a serious misunderstanding of the idea of the action of the hormones from the gonads and of hormones in general. No one would suggest that the hormones from the testis should be regarded as in any sense the origin of the antlers of a stag. If so, why should not antlers equally develop in the stallion or in the buck rabbit, or indeed in man? How far Doncaster is right in holding that the soma is different in the two sexes is a question already mentioned, but it is obvious that in each individual the somatic sexual characters proper to its species are present potentially in its constitution by heredity--in other words, as factors or determinants in the chromosomes of the zygote from which it was developed; but the normal development of such characters in the individual soma is either entirely dependent on the stimulus of the hormone of the gonad or is profoundly influenced by the presence or absence of that stimulus. The evidence, as we have seen, proves that, at any rate in the large number of cases where this relation between somatic sex-characters and hormones produced by the reproductive organs exists, the characters are inherited by both sexes. In one sex they are fully developed, in the other rudimentary or wanting. But the sex, usually the female, in which they are rudimentary or wanting is capable of transmitting them to offspring, and also is capable of developing them more or less completely when the ovaries are removed, atrophied or diseased.