122 ANALYSIS OF THE FOUR PRINCIPLES (CONTINUED). 
the second case it is primarily separative, but inevitably passes into 
segregate breeding. Divergence through diversity of use, and the 
resulting diversity of acquired characters, whether inherited or not, 
will operate as surely in the one case as in the other. 
Sustentational csolation 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 phyto- 
phagic varieties (as they are sometimes called) of insects exist, a con- 
siderable 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 offspring 
of other varieties; and it is evident 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 min- 
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. 
Protectional isolation is isolation 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 mem- 
bers of the one species; and the use of these different methods must 
sometimes result in segregating 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 butter- 
fly 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. 
Nidificational isolatton.—Let us now consider the effects of diver- 
gent 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 environ- 
