134 THE AMERICAN NATURALIST [Vol. XL VII 



the genera, are represented by but one species each; 

 seven are represented by from two to ten species ; three 

 are represented by from twenty to thirty species, three by 

 from forty to sixty-five species, one by one hundred and 

 fifty species, one by two hundred, and three by from two 

 hundred and twenty-five to two hundred and fifty species. 

 Thus while sixteen genera contain less than ten species 

 each, four genera contain more than two hundred species 

 each. 



This extraordinary condition of the species in their re- 

 lation to each other presents an attractive problem. 

 What is its significance? What are indeed the special 

 conditions influencing species-forming within the group? 



From many years' work with these insects, including 

 the description of several hundred new species, and the 

 examination of long series of individuals of species in 

 several different genera, I can say with confidence that 

 the evolutionary factor of isolation plays a conspicuous 

 part in Mallophagan species-forming. One soon comes 

 to the acceptance of a very flexible species description for 

 any given Mallophagan kind. While the score of indi- 

 viduals of one kind that one may collect from a single 

 host individual will agree well with each other as to 

 details of structure and pattern, the specimens of the 

 same kind from another host individual of the same host 

 species collected either in the same locality or a distant 

 one, and the specimens from a third host individual and 

 from a fourth and fifth, and so on, will all show many 

 obvious, if mostly small, variations from the specimens 

 taken from the first or any other host individual. 



That is, each host individual is, in a way, a small island, 

 biologically considered, with its inhabitants more or less 

 nearly completely isolated from the inhabitants of other 

 host islands. So that each species is made up of many 

 dislocated small groups which may have, as when they 

 are on birds of solitary habit, but little opportunity for 

 mixing and cross-breeding with the members of the spe- 

 cies-body as a whole. The group on one host bird may 



