THROUGH CUMULATIVE SEGREGATION. 
227 
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-bnilcling 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 exist- 
ing, 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 en- 
vironment 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 chaDged their inherited adaptations to 
meet the new habits of nest-buildiug and of feeding. It is also 
evident that the prevention of free interbreeding 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 separates an individual from the mass of the species will 
necessarily leave it alone, without any chance of finding a consort 
that may join in producing a new intergeuerant. The answer is 
that there is no such necessity. Though it may sometimes 
happen that an individual may be separated from all companions 
by its industrial habit, it is usually found that those that at one 
time and one place adopt the habit are usually sufficient to keep 
up the new strain, if they succeed in securing the needed sus- 
tenance. 
( i ) Chronal Segregation 
is Segregation arising from the relations in which the organism 
stands to times and seasons. 
I distinguish two forms — Cyclical and Seasonal Segregation. 
4. Cyclical Segregation is Segregation arising from the fact 
that the life cycles of the different sections of the species do not 
mature in the same years. 
A fine illustration of this form of Segregation is found in the 
case of Cicada septemdecim, whose metropolis is in Virginia, 
Maryland, and Delaware, though many out-lying broods are 
found in other regions east of the Mississippi River. The typical 
form has a life-cycle of seventeen years, but there is a special 
