DIVERGENT EVOLUTION THROUGH SEGREGATION. 301 
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 constitu- 
tional 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 separation passes into segrega- 
tion. The Fall River brood of Cicada septemdecim being entirely separ- 
ated from all other broods of the same race by being belated a year 
may be modified by forms of natural selection that never arise in these 
other broods. And this may be the case even if a brood observing the 
ordinary time is always associated with it in locality. 
5. Seasonal segregation is produced whenever the season for repro- 
duction in any section of the species is such that it can not interbreed 
with other sections of the species. It needs no argument to show that 
if in a species of plant that regularly flowers in the spring, there 
arises a variety that regularly flowers in the autumn, it will be pre- 
vented from interbreeding with the typical form. The question of chief 
interest is, Under what circumstances are varieties of this kind likely 
to arise? Is acasual sport of this kind likely to transmit to subse- 
quent generations a permanently changed constitution? If not, how 
is the new constitution acquired? One obvious answer is that it may 
arise under some special influence of the environment upon members 
of the species that are geographically or locally segregated from the 
rest of the species. 
But may not the variation in the season of flowering be the cause of 
segregation that will directly tend to produce greater variation in that 
respect in the next generation, and so on till the divergence in the con- 
stitutional adaptation to season is carried to the greatest extreme that 
is compatible with the environment? I believe that it not only may 
but must have that effect; but we should remember that the average 
form which flowers at the height of the season will so vastly predomi- 
nate over the extreme forms that the latter will be but stragglers in 
comparison. 
In regard to the one point of the season of readiness for propagation 
this principle is segregative; but in other respects it is simply separa- 
tive, unless through the principle of correlated variation other char- 
acters are directly connected with the constitution that determines the 
season. It will be observed that seasonal segregation is produced by 
a parallel and simultaneous change in the constitution of members in 
one place sufficient to propagate the species; while cyclical segregation 
is produced by a simultaneous acceleration or retardation in the devel- 
opment of members in one place sufficient to propagate the species 
without disturbing the regular action of the constitution under ordi- 
nary circumstances. 
