THOUGH Prof. Goldschmidt's treatise on sex-determination is in scope similar to the textbooks published by Doncaster and by Morgan in 1912, knowledge has increased so rapidly since then that there is plenty of room for a new statement. Moreover, as the author has himself devoted several years to the study of a special case which departs from the ordinary rules, his views will be of interest to geneticists. Up to a point, the mechanism of sex-determination is clear. On the one hand, we know that in several birds and some Lepidoptera the female is heterozygous in sex, but we have equally sound proof that in man and in several insects other than Lepidoptera the condition is reversed, the female being homo-zygous and the male heterozygous in respect of the sex-factor. The evidence for these conclusions is mainly either genetical or cytological. With the exception of Drosophila, which, after some doubt, observers have agreed to regard as having the male XY and the female XX, there is no specific form in which positive evidence of both kinds, genetical as well as cytological, can yet be produced. The absence, however, of such convergent testimony need not trouble us at this stage, for each class of proof is by itself adequate so far as it goes. On the whole, also, though difficulties are met with in special examples, the evidence from operative and other collateral observations agrees well with the conclusions deduced from genetical and, cytological sources. Mechanismus und Physiologie der Geschlechts-bestimmung. By Prof. Richard Goldschmidt. Pp. viii + 251. (Berlin: Gebruder Borntraeger, 1920.) Price 32 marks.