180 DARWINISM 



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 are 

 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 



