42 THE PROBLEMS OF EVOLUTION 



portance in our consideration of species save in 

 showing that variable conditions in nature may be 

 definitely responsible for the appearance of differ- 

 ent individual characteristics. The fact that dif- 

 ferent conditions obtain in different regions is more 

 important. When the gradual dispersal of a species 

 brings it into contact with these variable condi- 

 tions they may play a part in the production of 

 geographic races. The part which they play must 

 be considered later; it is a disputed one, but the 

 races concerned are real and important. 



Many species of wide distribution encounter 

 diflferent environmental conditions in various parts 

 of their range, and may show marked diversity of 

 individual characters correlated with regional oc- 

 currence. John B. Smith, one of the early students 

 of North American Noctuidae, described many 

 species of these obscurely colored moths which 

 have since been shown to associate with each other 

 by gradual transition through the intervening re- 

 gions. A fairly simple case of this nature is that of 

 the common checkered skipper, Pyrgus tessellata. 

 Skinner ^^ named the pale western form of this 

 species ocddentalis, Williams ^^ later discovered 

 that the two differed in genitalic structure and 

 called them good species. Barnes and Lindsey " 

 examined material from various localities scattered 



" Ent. News, Vol. XVII, p. 96, 1906. 

 " im.. Vol. XXX, p. 38, 1919. 

 " Ibid., Vol. XXXII, p. 79. 1921. 



