228 
REV. J. T. GULTCK ON DIVERGENT EVOLUTION 
race ( Cicada tredecim, Riley) tliat is separated from the 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 Tail 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-River brood of Cicada 
* See statement by Prof. C. Y. Riley, in 1 Science,’ vol. vi. p. 4. For par- 
ticulars concerning the distribution and habits of this species see a paper by 
Prof. Riley, read before tbe Biological Society' of Washington, May 30, 1885, 
extracts from which are given in ‘ Science,’ vol. v. p. 518. 
