SEXUAL AND CYCLICAL ISOLATION. 233 



38, and most of those occupied by the thirteen-year race lie south of 

 that line, though in Illinois there is a thirteen-year brood as far north 

 as latitude 40. Dr. Riley has not referred to the coincidence, but it 

 seems to me a fact of some interest in this connection that the southern 

 limit of the great ice-cap which covered Canada and the northern part 

 of the United States during the glacial epoch extended along an irreg- 

 ular line between the parallels of latitude 38 and 40. Lying south 

 of the ice region there was probably a considerable belt of country cov- 

 ered with pines and other conifers not adapted to the breeding of this 

 species, so that both races, if they then existed, must have been 

 crowded into the southern portion of the region now occupied by the 

 thirteen-year race. 



Instinctive and cyclical forms of isolation, such as cause the inde- 

 pendent generation of the races and broods of this species, are usually 

 associated with clearly developed specific distinctions relating to form, 

 color, and function. This does not, however, prove that the isola- 

 tive divergence was subsequent to the general divergence in other re- 

 spects. The number of generations covered by the initial stage in 

 which the different sections are only races is very small compared with 

 those that are likely to be covered by the stages when they are separate 

 species and genera. It is only, therefore, by rare chance that we find two 

 forms that are still in the earliest stage of palpable divergence and are, 

 at the same time, completely segregated by constitutional differences. 

 Again, segregative endowments are usually developed somewhat grad- 

 ually; and while the segregation is advancing other transformations 

 take place, so that by the time all crossing has come to an end the dif- 

 ferent sections have become well-marked species. Sometimes, as in 

 the three species of butterflies already considered, there is more or less 

 crossing after the sections have become quite distinct species. Such 

 cases, however, as are presented by the thirteen-year and seventeen-year 

 races and by the different broods of this species of Cicada show that 

 complete segregation may be produced by the psychological and physio- 

 logical constitution of different races, while distinctions of form, color 

 and manner of call are entirely wanting so far as we can observe. 

 This has seemed impossible to some naturalists, especially since Dar- 

 win has admitted that cross-sterility can not be attributed to natural 

 selection, and has, therefore, attributed it to the indirect effects of 

 other qualities which have been produced by natural selection. 



The great contrast in this respect between the species of Basilarchia 

 and the thirteen-year and seventeen-year races of the periodical cicada 

 may perhaps be partially explained by the fact that the latter spend 

 the greater part of their existence under ground, where the conditions 



