CHRONAL ISOLATION. 123 
ment in two sections of the same species. One section would have 
retained the old habit of building in the cliffs, with all the old adapta- 
tions to the circumstances that depend on that habit, while another 
section of the species would have availed itself of the new opportuni- 
ties for shelter under the eaves of houses, and would have changed 
inherited adaptations to meet the new habits of nest-building 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 newintergenerant. ‘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 which at one time and in one place adopt the 
habit are usually sufficient to keep up the new strain if they succeed 
in securing the needed sustenance. 
4. Chronal Isolation. 
Chronal isolation is isolation arising from the relations in which the 
organism stands to times and seasons. 
I distinguish two forms—cyclical and seasonal isolation. 
Cyclical rsolation is isolation 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 isolation is found in the case of 
Cicada septendecim, whose habitat is the northern portion of the 
Mississippi Valley and of the Atlantic States, though many outlying 
broods are found in other regions. The typical form has a life-cycle 
of seventeen years, but there is a thirteen-year race (Cicada tredecim 
Riley) found chiefly in the Southern States, and therefore separated 
from the typical form, both locally and chronally. As the life- 
cycle of this race is thirteen instead of seventeen years, in a district 
where the habitats of the two races overlap, even if there were no 
physiological or psychological incompatibility to overcome, inter- 
breeding could occur between the two forms only once in 221 years, 
that is, once in 13 generations of the longer-lived race, and once in 
17 generations of the shorter-lived race. The distribution of the two 
races in different districts seems to indicate that local isolation under 
different climatic conditions has had an important influence in their 
development. It is manifest, however, that if during a period of 
