226 EET. J. T. OTJLICK ON" DIVERGENT EVOLUTION 



Evolution through Diversity of Use, will operate as surely in the 

 one case as in the other. 



1. Sustentational Segregation arises from the use of different 

 methods of obtaining sustentation by members of the same 

 species. 



There can be no doubt that of the innumerable cases where 

 phytophagic varieties (as they are sometimes called) of insects 

 exist, a considerable proportion would be found on investigation 

 to be permanent varieties producing offspring that are better 

 adapted to the use of the special form of food consumed by the 

 parents than are the offspring of other varieties ; and it is evi- 

 dent that if the peculiar habits of each variety had no tendency 

 to produce segregative breeding this result would not be reached ; 

 for each variety would be promiscuously mingled with every 

 other, and, though the tendency to variation might be greatly 

 increased, the regular production of any one variety of young 

 would be prevented. 



A large mass of facts could be easily gathered illustrative of 

 Sustentational Segregation ; but as the principle will probably 

 be denied by no one, we shall pass on without farther expansion 

 of this part of the subject. 



2. Frotectioncil Segregation is Segregation from the use of 

 different methods of protection against adverse influences in the 

 environment. 



When a new enemy enters the field occupied by any species 

 different methods of escape or defence are often open to the 

 members of the one species ; and the use of these different 

 methods must sometimes result in the segregation of the mem- 

 bers according to the methods adopted. Some may hide in 

 thickets or holes, while others preserve themselves b}^ flight. 

 Supposing the species to be an edible butterfly occupying the 

 open fields, and the new enemy to be an insectivorous bird also 

 keeping to the open country, certain members might escape by 

 taking to the wood-lands, while others might remain in their old 

 haunt, gaining through Protectional Selection more and more 

 likeness to some inedible species. 



3. Nidijicational Segregation. — Let us now consider the effects 

 of divergent habits in regard to nest-building. It is well known 

 to American ornithologists that the Cliff Swallow of the eastern 

 portions of the United States has for the most part ceased to 

 build nests in the cliffs that were the original haunts of the 



