66 ORGANIC EVOLUTION 



of the animals themselves." This is a good statement of the 

 case except that Lloyd Morgan should have said isolation 

 may at once arise, not " will " at once arise. 



Romanes 1 has called attention to. a factor in segrega- 

 tion which has as yet been insufficiently studied, but which 

 may prove of the greatest importance. He has called it 

 physiological selection. It has been observed that certain 

 individual animals of the same species, when crossed with 

 each other, are infertile, whereas either one, if crossed with 

 a different mate, might have been normally fertile. There 

 exists some insufficiently understood bar to fertility between 

 those two individuals. This is a restraint upon the perfect 

 freedom of intercrossing, a sort of negative segregation, and 

 must have a real effect on evolution. It seems quite pos- 

 sible that further observation and experiment may show 

 this factor in segregation to be more common and impor- 

 tant than, in our present ignorance of the actual facts, we 

 can assert. The reproductive function is very delicate and 

 liable to disturbance from apparently slight causes. Many 

 wild animals, however well kept, are barren in captivity or 

 are Jess fertile than when unrestrained. Transportation to 

 a strange locality sometimes interferes with reproduction. 

 Again, there are some observations which suggest that 

 variation in structure in any of the different organs of the 

 body may be correlated with such disturbance of the repro- 

 ductive functions as to decrease the fertility of crosses be- 

 tween the individuals which diverge from the species type 

 and those which do not so diverge. This point, however, 

 needs much more study before we can determine the im- 

 portance of its influence in producing physiological segre- 



1 Darwin and After Darwin, Volume III, "Isolation and Physiological Selection." 



