462 THE GERM-PLASM 



exhibited by human beings. It has long been known that 

 hcetiwphilia occurs in men only, but is transmitted by women. 

 If we assume that the visible sexual differences, as well as those 

 existing in the system which we are unable actually to recognise, 

 are due to the presence of double determinants, the peculiar 

 limitation of this uncommon disease to one sex is explained. 

 The disease, like a secondary sexual character, is only trans- 

 mitted to the sex in which it first appeared, for this half of 

 the double determinants of the • iiiesoblast germ ' has alone been 

 modified by the disease. 



The sexnal polymorphism exhibited by certain butterflies may 

 also be explained by assuming the presence of double deter- 

 minants of several local varieties of the same species which 

 interbreed with one another. The polymorphism of bees and 

 other animals which form communities, requires, however, the 

 assumption of triple or quadruple determinants. In these 

 animals the female half of the double determinant again becomes 

 doubled, and this may also be the case as regards the male half 

 (Termites). 



Lastly, the assumption of double determinants in the idio- 

 plasm accounts for temporary dimorphism, such as seasonal 

 dimorphism. 



The occurrence of dimorphism is in all cases attributable to 

 the presence of two kinds of determinants ; but the causes which 

 determine which of the two is to become active, are extremely 

 varied, and cannot in many cases be accurately indicated. 

 The determining influences, however, are always external ones 

 — such as fertilisation, nutrition, and the effect of light in cases 

 oi dichogeny in plants. 



It is self-evident from the theory of heredity here propounded, 

 that only those cliaracters are transmissible which have been 

 controlled — i.e., produced — by determinants of the germ, and 

 that consequently only those variations are hereditary which 

 result from the modification of several or many determinants 

 in tlie germ-plasm, and not those which have arisen subse- 

 quently in consequence of some influence exerted upon the cells 

 of the body. In other words, it follows from this theory that 

 somatogenic or acquired characters cannot be transmitted. 



This, however, does not imply that external influences are 

 incapable of producing hereditary variations ; on the contrary, 

 they always give rise to such variations when they are capable 



