HEREDITY OF SEX 207 
the case of certain plants it is the male which is hetero- 
zygous, whilst the female is homozygous. On the 
other hand, the first Mendelian interpretation of sex 
ever put forward—namely, that due to Castle— 
supposed that both sexes were heterozygous. Many 
further facts must be obtained and discussed’ before 
the problem of sex-determination can be regarded as 
in any degree settled. 
By way of further illustrating the far-reaching im- 
portance of the information which has been rendered 
available by the combined use of experimental and 
cytological methods, we may here briefly criticise the 
celebrated theory of inheritance put forward by Weis- 
mann in 1892 under the name of the ‘Germ Plasm 
Theory.’ Some notice of this theory, which might 
otherwise have been permitted to go the way of similar 
valuable provisional hypotheses, is rendered almost 
necessary by the circumstance of its having been 
recently revived in a prominent manner in the English 
translation of Weismann’s book, ‘The Evolution 
Theory.’ In this book, published in 1904, the bearing 
of the Mendelian evidence upon the subject of inheri- 
tance is practically ignored ; although, in the face of the 
definite experimental information now rendered avail- 
able, the younger biologists, at least, are beginning 
to realize that the circumstantial evidence, formerly 
so much relied upon, will. in future constitute a much 
less prominent feature in these discussions. 
Weismann’s theory of inheritance, and the Theory 
