228 



REV. J. T. GTJLTCK ON DIYERaENT EVOLUTION 



race {Cicada tredecion, Eiley) that is separated from tbe typical 

 form, both locally and chronally. As the life-cycle of this race 

 is thirteen instead of seventeen years, even if occupying the same 

 districts and breeding at exactly the same season, interbreeding 

 could occur between the two forms only once in 221 years, or 

 once in 13 generations of the longer lived race, and once in 17 

 generations of the shorter lived race. During the year 1885 the 

 two races appeared simultaneously. The opportunity for testing 

 whether they would freely interbreed if brought together has, 

 therefore, passed not to return till the year 2106 ; but the dis- 

 tribution of the two races in different districts seems to indicate 

 that Local Segregation has had an important influence in the 

 development of the race. It is manifest, however, that if during 

 a period of local separation, or if during the period of 221 years 

 of Cyclical separation after the thirteen-year race was first 

 formed, this race should become modified in the season of its 

 appearing, there would after that be no mingling of race, though 

 brought together in the same districts. This would be Seasonal 

 Segregation, which we shall consider in the next section ; but 

 what is of special interest here, as an example of complete 

 Cyclical Segregation, is the fact that at Fall River, Massachu- 

 setts, there is a brood of the septemdecim form, due a year later 

 than the universal time of appearing.* 



In any species where the breeding of each successive genera- 

 tion is separated by an exact measure of time which is very 

 rigidly regulated by the constitution of the species, Cyclical 

 Segregation will follow, if, through some extraordinary combina- 

 tion of circumstances, members sufficient to propagate the species 

 are either hastened or delayed in their development, and thus 

 thrown out of synchronal compatibility with the rest of the 

 species. If, after being retarded or hastened in development so 

 that part of a cycle is lost or gained, the old constitutional time 

 measure reasserts itself, the Segregation is complete. 



So far as this one point relating to the time of maturing is 

 concerned the constitutional difference is segregative, while in 

 every other respect it will be simply separative, except as sepa- 

 ration passes into Segregation. The Fall-Eiver brood of Cicada 



* See statement by Prof. C. V, Kiley, in ' Science,' vol. vi. p. 4. For par- 

 ticulars concerning the distribution and habits of this species see a paper by 

 Prof. Riley, read before the Biological Society of Washington, May 30, 1885, 

 extracts from which are given in ' Science/ vol. v. p. 518. 



