DISTRIBUTION OF BUTTERFLIES IN THE NEW WORLD 363 



behri is located in the southern Sierra Nevada from its presently 

 nearest relative in central Oregon. Many other examples of such 

 disjunction could be cited. 



Tropical forms occasionally invade more northern areas, where 

 they may stay for a season or two. Extremes of climates are usually 

 the limiting factors controlling distributions, but they do not always 

 occur annually. Thus colonies can often survive out of their more 

 permanent range for several years until an unusual year wipes them 

 out. Colias eurytheme periodically invades the colder regions of 

 northern Canada where it may survive for a number of generations. 

 Then a cold winter kills all individuals. Phoebis periodically invades 

 northern areas during the summer and may even survive mild 

 winters. 



Ice age distributions may or may not have been very different 

 from what they are now, but great areas were undoubtedly denuded 

 of all populations by the glaciers. Such areas are still being repopu- 

 lated. Some of the reinvasions from opposite sides of the former 

 continental glacier of North America have led to interesting read- 

 justments of species relationships. Colias hecla and Colias nastes, for 

 example, hybridize in that area to form a third species, not found 

 elsewhere. There is often a narrow line of demarcation between two 

 species, one that existed south and east of the glacier, and one that 

 held out in the unglaciated area to the northwest. For example, in 

 Limenitis, the two species astyanax (south) and arthemis (north) now 

 narrowly overlap in the Great Lakes area. Similarly, Colias interior 

 (south) and Colias palaeno (north) narrowly overlap across central 



Canada. 



Colias eurytheme is absent from the Gulf Coast of the eastern 

 United States in summer, since the temperatures there are unsatis- 

 factory for its existence. However, it is reestablished by migrants 

 from the north in autumn, and passes through two or three genera- 

 tions before the hot summers arrive again. In the meantime, cold 

 winter weather forces the adults to disappear for a month or two. 



Daily differences in activity and distribution have been indicated 

 for the genetically determined color phases of Colias. It is of course 

 the selective effect of climate on individuals that determines in a 

 broader sense the distribution of species, of genera, and of families. 

 The groups are no more important than the individuals of which 

 they are composed. 



