DIMORPHISM AND POLYMORPHISM 373 



In certain instances, polydactylism is known to be transmitted 

 to the male members of a family only, while in others it passes 

 from the mother to the daughters exclusively. 



It appears, however, that such modifications of the one half 

 in the double-determinants may in the course of time be trans- 

 ferred to the other half, even though, in the first instance, this 

 only occurs to a slight extent ; for cases are known in which 

 an abnormality first arose (?) in the male sex, and afterwards 

 passed over to certain individual female descendants. These 

 cases have certainly not been followed out with sufficient accu- 

 racy to enable us definitely to deny that a modification of both 

 halves of the double-determinants in question might possibly 

 have taken place from the first, and that it only affected those 

 in the one half (the homologous determinants) in a smaller 

 number of ids. 



Numerous instances in which an abnormality appears, some- 

 times in the male, and sometimes in the female members of a 

 family, prove that both halves may become modified at the same 

 time. Prosper Lucas mentions several instances of this kind, such 

 as that of the family Ruhe : — in the first generation observed, 

 the mother transmitted her polydactylism to the daughter, and 

 in the second this peculiarity was passed on from the mother to 

 the son, while in the third it was transmitted from the father to 

 the son. Numerous facts in zoology indicate the correctness of 

 the assumption that modifications in one half of a double-deter- 

 minant exert an influence on the other half, so as to result in a 

 similar transformation. It is well known that many secondary 

 sexual characters of the male in birds and insects appear in a 

 slighter degree in the female. Amongst the LycceiiidcE — which 

 are called ' blues,' on account of the preponderance of this 

 colour in the members of the family — some species exist in 

 which both sexes are brown, while in most the male is blue and 

 the female brown, and in a small number of southern species, 

 again, both sexes are blue. There can be no doubt that brown 

 was the original colour of these species, and that the blue tint first 

 appeared in the males, while the females remained brown ; and 

 that, finally, in certain species, the females also became blue, 

 although not so markedly so as the corresponding males. The 

 males therefore preceded the females as regards the change of 

 colour; and if, with Darwin, we attribute the impulse to this 

 change to sexual selection, it follows that the blue colour of the 



