DIVERGENT EVOLUTION THROUGH SEGREGATION, 299 
gled 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 susten- 
tational segregation; but as the principle will probably be denied by 
no one, we should pass on without further expansion of this part of 
the subject. 
2. Protectional 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 defense 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 members according to the methods 
adopted. Some may hide in thickets or holes,while others preserve 
themselves by 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 woodlands, while others might remain in their old 
haunt, gaining through protectional selection more and more likeness 
to some inedible species. 
53. Nidificational 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 species, and has availed itself 
of the protection from the weather offered by the eaves of civilized 
houses; and that with this change in nest-building has come a change 
in some of its other habits. Now there is reason to believe that if the 
number of houses had been limited to a hundredth part of those now 
existing, and if that limited number had been very slowly supplied, 
this gradual change in some of the elements of the environment would 
have resulted in divergent forms of adaptation to the environment in 
two sections of the same species. One section would have retained the 
old habit of building in the cliffs, with all the old adaptations to the 
circumstances that depend on that habit; while another section of the 
species would have availed itself of the new opportunities for shelter 
under the eaves of houses, and would have changed their inherited 
adaptations to meet the new habits of nest-building and of feeding. 
It is also evident that the prevention of free inter-breeding between the 
different sections caused by the diversity of habits would have been 
an essential factor in the divergence of character in the sections. 
It simply remains to consider whether the industrial habit that sepa- 
rates an individual from the mass of the species will necessarily leave 
it alone, without any chance of finding a consort that may join in pro- 
ducing a new intergenerant. The answer is that there is no such 
necessity. Though it may sometimes happen that an individual may 
