360 Evolution and Adaptation 



the evolution of such color, or that it has been acquired 

 through a life and death struggle of the individuals of the 

 species. 



Sexual Dimorphism 1 and Trimorphism 



It has been found in a few species of animals and plants 

 that two or more forms of one sex may exist, and here we 

 find a condition that appears to be far more readily explained 

 on the mutation theory than on any other. The most impor- 

 tant cases, perhaps, are those in plants, but there are also 

 similar cases known amongst animals, and these will be given 

 first. 



There is a North American butterfly, Papilio tiimus, that 

 appears under at least two forms. In the eastern United 

 States the male has yellow wings with black stripes. There 

 are two kinds of females, one of which resembles the male 

 except that she has also an orange " eye-spot " ; the other 

 female is much blacker, and this variety is found particularly 

 in the south and west. The species is dimorphic, therefore, 

 mainly in the latter regions. 



The cases of seasonal dimorphism offer somewhat similar 

 illustrations. The European butterfly, Vanessa levana-prorsa, 

 has a spring generation ilevana) with a yellow and black pat- 

 tern on the upper surface of the wings. The summer genera- 

 tion (prorsa) has black wings " with a broad white transverse 

 band, and delicate yellow lines running parallel to the margins." 

 These two types are sharply separated, and their differences 

 in color do not appear to be associated with any special pro- 

 tection that it confers on the bearer. These facts in regard 

 to Vanessa seem to indicate that differences may arise that 

 are perfectly well marked and sharply defined, which yet 

 appear to be without any useful significance. 



1 This term is used here in the sense employed by Darwin. The same term is 

 sometimes used for those cases in which the male departs very greatly from the 

 female in form. 



