360 THE GERM -PLASM 



be of any use to the species, natural selection can have taken no 

 part in the addition of male determinants to the somatic cells 

 of the female body, or vice versa. Such a transference must 

 therefore depend on an unintentional secondary effect of existing 

 arrangements and forces. I soon, however, recognised that 

 such arrangements actually exist, and that the assumption of 

 mechanically unseparable double-determinants is not necessary 

 in order to explain the presence of the two sexual determinants 

 in the region where one of them undergoes development. I 

 therefore attach no value to the idea of the material connection of 

 the two dimorphic halves of the primary determinants in question. 

 In fact I shall have occasion to show presently that these halves 

 imist, in any case, sooner or later become separated as inde- 

 pendent determinants in the course of phyletic development. 



The reason that such double determinants must, however, 

 always remain close together, even after their separation, results 

 simply from the mechanism for the ontogeny of the idioplasm, 

 which we suppose to consist in the gradual disintegration of 

 the mass of determinants of the germ-plasm into smaller groups. 

 They divide according to a definite law into smaller and smaller 

 groups in the course of the embryonic cell-divisions. None of 

 them remain unused or undergo destruction, but each passes 

 through a definitely prescribed course, and the determinants 

 for any particular part or region of the body 7tiust necessarily 

 remain together, even when they are not inseparably connected 

 mechanically. In 2. physiological s,Q.n?,Qj therefore, they are still 

 dotible determinants, — i.e., each half controls the same region, — 

 and in this sense I shall now use the term. 



A transfer of the secondary sexual characters, such as occurs 

 in birds, cannot take place in the LyccenidcB., because the wing- 

 scales are never formed more than once in the course of life, 

 and consequently we have no means of proving the presence 

 of double determinants in the cells of the wing. Other ob- 

 servations on insects indicate, however, that their idioplasm is 

 nevertheless capable of producing such a sexual transference. 



This is especially shown in the case of the occasional herma- 

 phrodite forms of insects, the most instructive instance — which 

 has been very accurately investigated by Leuckart* and von 



* R. Leuckart, ' Sitzungsberichte der deutschen Naturforscherver- 

 sammlung,* 1864. 



