THE IMPORTANCE OF IMPREGNATIONAL ISOLATION. 95 
frequently than others. In a memoir on Regression, Heredity, and Panmixia, 
recently published, I have ventured to term this possible factor of progressive 
evolution “‘ reproductive selection.” 
This definition seems very definitely to include not only sexual 
selection but the forms of reflexive selection which I have called social, 
filio-parental, dominational, and impregnational selection. But we 
can have but little confidence that we have reached the correct inter- 
pretation of the meaning he would have us give to the terms “‘ repro- 
ductive selection "’ and “‘fertility,’”’ for when we come to the con- 
cluding sentence of the chapter, on page 102, we read: ‘‘In civilized 
man the survival of the fittest appears to be replaced by the survival 
of the most fertile,’’ which seems to imply that fertility as he uses 
it does not depend on fitness. 
13. The Importance of Impregnational Isolation. 
The prevention of crossing between groups produced by the dif- 
ferent forms of impregnational isolation is connected with several 
problems of great interest. After referring to the terms segregate 
survival, segregate union, and “ physiological isolation,’’ under which 
some of these principles have been grouped, we will consider certain 
of these problems. 
(1) Segregate survival has presented itself to my mind in five aspects, 
namely: Segregate fecundity, vigor, adaptation, freedom from compett- 
tion, and escape from enennes. 
These are the influences that give emphasis to the importance of 
any form of positive segregation by which those of any one kind are 
brought together and enabled to breed together. It may at first seem 
that these are simply the forms of selection that are producing trans- 
formation within the different intergenerating groups. It is, how- 
ever, quite otherwise; for diversity of selection may exist in full 
force in two adjoining districts, and partial positive segregation may 
exist between the two groups of a species occupying these districts; 
but, if the mixed unions are as fertile as the pure unions, and produce 
young as successful in surviving as those produced by the pure unions, 
the probability is that the two groups will not become increasingly 
divergent. 
Impregnational isolation has now been presented under eight forms, 
of which the first three rest on morphological and physiological incom- 
patibilities preventing or interfering with mixed unions, which may, 
therefore, be called segregate union. The second group of five forms 
rests on incompatibilities preventing either the normal fruitfulness of 
mixed unions or the power of the young thus produced to reach the 
ratio of individual survival and reproduction reached by the young of 
