DIMORPHISM AND POLYMORPHISM 359 



male bird, nor do they crow ; and conversely, when hens 

 become sterile from age, or if their ovaries become degenerated, 

 they take on the external sexual characters of cocks. I possess 

 a duck which no longer lays eggs, and has assumed the colora- 

 tion of the drake. Men who have been castrated in their youth 

 retain a high voice like that of the other sex, and the beard does 

 not become developed. 



These facts obviously compel us to assume that the capacity 

 for the development of secondary female characters exists in a 

 latent condition in the body of the male, and vice versa, and 

 that these are ready to undergo development under certain con- 

 ditions. Darwin also arrived at the same conclusion. The only 

 argument which might be advanced against its correctness is 

 that a change of secondary sexual characters in any particular 

 individual has only been observed in rare instances, and in very 

 few species of the higher animals, such as birds and mammals. 

 It might, therefore, be doubted whether it is possible to draw 

 a general conclusion from such isolated observations. These 

 instances nevertheless remain to be accounted for, and we must 

 attempt to explain them in accordance with our theory. 



Let us take a very simple example. In many butterflies of 

 the family Lyccenidce, the upper surface of the wings is brown in 

 the females, and blue in the males ; and it seems very probable 

 that brown was the original colour, as species of LyccEua exist 

 at the present day in which both sexes are of this colour. 

 The process which took place in the idioplasm in order that 

 the change of colour may have occurred, must have consisted 

 in the primary determinants of those cells which decide the 

 colour of the wings — which we will speak of as ' brown ' deter- 

 minants — becoming transformed into ' blue ' determinants in the 

 germ-plasm, this change only occurring after they had become 

 doubled and in such a manner that only one of the twin-deter- 

 minants in each case remained brown, an arrangement also taking 

 place which only allowed each to become active alternately. 

 We thus arrive at the assumption of double detertninaiits, just 

 as in the case of the determination of sex in the germ-cells. 

 I at first believed that it was indispensable to assume the 

 existence of determinants with different halves, merely because 

 the presence of inactive determinants in the corresponding 

 part of the body could not be otherwise explained. As the 

 adoption of the characters of the opposite sex cannot possibly 



