SUMMARY AND CONCLUSION 461 



presence in the idioplasm of double determinants for all those 

 cells, groups of cells, and entire organisms, which are capable 

 of taking on a male and female form. But only one half of such 

 a double determinant remains inactive, while the other becomes 

 active. The sexual differentiation of the germ-cells must thus be 

 due to the presence of spermatogenetic and oogenetic double 

 determinants ; and even all the secondary sexual characters must 

 be traced to a similar origin in the idioplasm. The corresponding 

 double determinants are contained not only in the germ-plasm, 

 but are passed on through the cell-stages of ontogeny to that 

 part of the body in which the two characters become separated 

 from one another. One of the determinants then becomes 

 active, its twin half remaining in an inactive condition in the 

 nucleus of a somatic cell, and under certain circumstances 

 becoming active subsequently. This, however, only occurs 

 exceptionally, in such cases as that in which a female animal 

 {e.g.^Ti hen or duck) develops male characters in consequence 

 of castration. Hermaphrodite bees, in which the whole body 

 consists of the most wonderful intermixture of male and female 

 parts, furnishes an instructive proof of the presence of both 

 kinds of characters in all parts of the body, and consequently 

 of the truth of the assumption of double determinants. 



Double determinants not only occur individually, but entire 

 groups of male and female determinants are opposed to one 

 another, and these are just as dependent on one another as are 

 the two halves of the individual double determinant, one of 

 which always remains inactive when the other becomes active. 

 These groups may be very dissimilar; in many cases {e.g., the 

 olfactory organs of male crustaceans and the ornamental 

 feathers of male birds) the male group contains many more 

 individual determinants than the female. One half of the 

 double group may also become degenerated, so that the corre- 

 sponding organ {e.g., the wing in many female butterflies) 

 disappears in one sex. 



The number of double determinants reaches its highest limit 

 when the two sexes differ completely from one another in all 

 their parts, as is the case in Bonellia 7'iridis; even then, how- 

 ever, a number of single determinants may still be present, if, as 

 in this case, the larval stage is similar in both sexes. 



The assumption of double determinants is also able to throw 

 some light upon certain enigmatical phenomena of heredity 



