180 
DARWINISM 
CIIAP. 
Physiological Selection. 
Another form of infertility has been suggested by Professor 
G. J. Romanes as having aided in bringing about the char¬ 
acteristic infertility or sterility of hybrids. It is founded on 
the fact, already noticed, that certain individuals of some 
species possess what may be termed selective sterility—that is, 
while fertile with some individuals of the species they arc 
sterile with others, and this altogether independently of any 
differences of form, colour, or structure. The phenomenon, 
in the only form in which it has been observed, is that of “in¬ 
fertility or absolute sterility between two individuals, each of 
which is perfectly fertile with all other individuals;” but Mr. 
Romanes thinks that “it would not be nearly so remarkable, or 
physiologically improbable, that such incompatibility should run 
through a whole race or strain .” 1 Admitting that this may be 
7. In the other part of the area, however, where hybridism occurs with 
perfect freedom, hybrids of various degrees may increase till they equal or 
even exceed in number the pure species—that is, the incipient species will 
be liable to be swamped by intercrossing. 
8. The first result, then, of a partial sterility of crosses appearing in one 
part of the area occupied by the two forms, will be—that the great majority 
of the individuals will there consist of the two pure forms only, while in the 
remaining part these will be in a minority, — which is the same as saying that 
the new physiological variety of the two forms will be better suited to the 
conditions of existence than the remaining portion which has not varied 
physiologically. 
9. But when the struggle for existence becomes severe, that variety which 
is best adapted to the conditions of existence always supplants that which is 
imperfectly adapted ; therefore, by natural selection the varieties which are 
sterile when crossed will become established as the only ones. 
10. Now let variations in the amount of sterility and in the disinclination 
to crossed unions continue to occur—also in certain parts of the area : exactly 
the same result must recur, and the progeny of this new physiological variety 
will in time occupy the whole area. 
11. There is yet another consideration that would facilitate the process. 
It seems probable that the sterility variations would, to some extent, concur 
with, and perhaps depend upon, the specific variations ; so that, just in propor¬ 
tion as the two forms diverged and became better adapted to the conditions of 
existence, they would become more sterile when intercrossed. If this were 
the case, then natural selection would act with double strength ; and those 
which were better adapted to survive both structurally and physiologically 
would certainly do so. 
1 Cases of this kind are referred to at p. 155. It must, however, be noted, 
that such sterility in first crosses appears to be equally rare between different 
species of the same genus as between individuals of the same species. Mules 
and other hybrids are freely produced between very distinct species, but are 
