370 EEV. J. T. GULICK ON 



an irregular line between the parallels of lat. 38° and 40°. Lying 

 south of the ice-region there was probably a considerable belt of 

 country covered 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 iuto the southern portion of the region 

 now occupied by the 13-year race. 



Instinctive and Cyclical forms of Segregation, such as cause 

 the independent generation of the races and broods of thiis species, 

 are usually associated with clearly developed specilic distinctions 

 relating to form, colour, and function. This does not, however, 

 prove that the segregative divergence was subsequent to the 

 general divergence in other respects ; for if complete segregation 

 continues for many generations it is likely to be followed by other 

 divergences, and the divergent forms are then ranked as separate 

 species. Moreover, 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 divergence and are, at the same time, com- 

 pletely segregated by constitutional differences. Again, segre- 

 gative endowments are usually developed somewhat gradually ; 

 and while the segregation is advancing other transformations 

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

 the different sections have become well-marked species. Some- 

 times, as in the case of 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 13- and 17-year races and by the different broods 

 of this species of Cicada., show that complete segregation may be 

 produced by the psychological and physiological constitution of 

 different races, while distinctions of form, colour, and manner 

 of call are entirely wanting so far as we can observe. This has 

 seemed impossible to some naturalists, especially since Darwin 

 has admitted that cross-sterility cannot 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 sjoecies of Basi- 

 larcliia and the 13- and 17-year races of Cicada septemdecim may 

 perhaps be partially explained by the fact that the latter spend 

 the greater part of their existence under ground, where the con- 



