374 THE GERM-PLASM 



females must have been introduced mechanically, owing to its 

 previous existence in the males, and that this secondary sexual 

 character affected the other sex in the course of a great number 

 of generations. According to our theory, this must have 

 been due to the modification of the • male ' halves of the 

 determinants having gradually caused a similar, if less marked, 

 modification of the 'female' halves. We can thus also to some 

 extent understand how it has been possible for certain females 

 to precede the others as regards this modification, as the 

 influence which produced it would not take effect to the 

 same extent and at a similar rate in all individuals. In 

 many species of Lycanidce in which the females are brown, 

 individuals of this sex occur more or less frequently which are 

 clouded with blue, or even exhibit this colour in a marked 

 degree. 



'&' 



3. Polymorphism 



Sexual trimorphisvt is of frequent occurrence in the animal 

 kingdom. I will here refer to certain instances amongst butter- 

 flies, which were first discovered by Alfred Russel Wallace, and 

 will begin with a case in which apparently the first step towards 

 polymorphism has been taken. 



The male of a common North American butterfly, Papilio 

 iitrnus, resembles the ordinary 'swallow-tail,' having yellow 

 wings ornamented with black transverse stripes ; while the 

 females sometimes resemble the males, and are sometimes quite 

 black, and may thus differ markedly from one another. Tlie 

 yellow females occur in the eastern and northern parts of the 

 United States, and the black ones in the west and south ; we 

 must therefore suppose that two local varieties of this butterfly 

 exist, in the northern of which the two sexes have a similar 

 coloration, while the southern form is dimorphic. This indi- 

 cates, in terms of the idioplasm, that the determinants for the 

 winsf-scales are single in the northern, and double in the southern 

 variety. Describing these determinants according to the colour 

 which they produce, we may say that the last-named variety 

 possesses double-determinants, the • male ' half of which is 

 'yellow' and the 'female' half 'black' ; while the single-deter- 

 minants of the northern form are 'yellow.' This species is 

 properly speaking, therefore, not trimorphic, but includes two 

 local varieties, one of which is dimorphic. If the two varieties 



