CHRONAL ISOLATION. 1 23 



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 new intergenerant. 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 isolation 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 Hfe-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-Uved race, and once in 

 1 7 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 



