SECONDARY SEXUAL CHARACTERS 129 



these types have suddenly appeared as mutants many 

 of the difficulties of the older theories are avoided, 

 and that such an assumption is in harmony with 

 an ever increasing body of evidence concerning 

 variation and heredity. On this view "natural se- 

 lection" plays no part in the formation of these 

 polymorphic forms," nor does sexual selection. The 

 absence of transitional forms is explicable on this 



T>"f 



Fig. 66. — Colias philodice, showing two female forms above and 

 one male form below. 



view, and unaccountable on the other theory. In 

 fact polymorphic forms, if they appear, would be 

 expected to persist if they are not harmful to the 

 species. 



We have in this country several species of butter- 

 flies in which polymorphism exists. In the north 

 the species Papilio turnus (Fig. 65) is alike in the male 

 and in the female. But in the south two types of 

 females exist — one like the male and the other a 

 black type. 



