152 Evolution and Adaptation 



" At one time it appeared to me probable, as it has to 

 others, that the sterility of first crosses and of hybrids might 

 have been slowly acquired through the natural selection of 

 slightly lessened degrees of fertility, which, like any other 

 variation, spontaneously appeared in certain individuals of 

 one variety when crossed with those of another variety. 

 For it would clearly be advantageous to two varieties or in- 

 cipient species, if they could be kept from blending, on 

 the same principle that, when man is selecting at the same 

 time two varieties, it is necessary that he should keep them 

 separate. 



" In considering the probability of natural selection having 

 come into action, in rendering species mutually sterile, the 

 greatest difficulty will be found to lie in the existence of 

 many graduated steps from slightly lessened fertility to abso- 

 lute sterility. It may be admitted that it would profit an 

 incipient species, if it were rendered in some slight degree 

 sterile when crossed with its parent form or with some other 

 variety ; for thus fewer bastardized and deteriorated offspring 

 would be produced to commingle their blood with the new 

 species in process of formation. But he who will take the 

 trouble to reflect on the steps by which this first degree of 

 sterility could be increased through natural selection to that 

 high degree which is common with so many species, and 

 which is universal with species which have been differentiated 

 to a generic or family rank, will find the subject extraordi- 

 narily complex. After mature reflection it seems to me that 

 this could not have been effected through natural selection. 

 Take the case of any two species which, when crossed, pro- 

 duced few and sterile offspring; now, what is there which 

 could favor the survival of those individuals which happened 

 to be endowed in a slightly higher degree with mutual infer- 

 tility, and which thus approached by one small step toward 

 absolute sterility ? Yet an advance of this kind, if the theory 

 of natural selection be brought to bear, must have inces- 



