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! 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, Papzlio turnus, 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 (/evana) 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. 
