274 CAUSES OF THE STERILITY 



selection as to that of special creation that in reciprocal 

 crosses the male element of one form should have been ren- 

 dered utterly impotent on a second form, while at the same 

 time the male element of this second form is enabled freely 

 to fertilize the first form ; for this peculiar state of the 

 reproductive system could hardly have been advantageous to 

 either species. 



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 

 absolute 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 off- 

 spring 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 selec- 

 tion to that high degree which is common with so man}' spe- 

 cies, and which is universal with species which have been 

 differentiated to a generic or family rank, will find the 

 subject extraordinarily 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, produced 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 infertility, 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 incessantly occurred with many species, for 

 a multitude are mutually quite barren. With sterile neuter 

 insects we have reason to believe that modifications in their 

 structure and fertility have been slowly accumulated by 

 natural selection, from an advantage having been thus indi- 

 rectly given to the community to which they belonged over 

 other communities of the same species ; but an individual 

 animal not belonging to a social community, if rendered 

 slightly sterile when crossed with some other variety, would 

 not thus itself gain any advantage or indirectly give any 

 advantage to the other individuals of the same variety, thus 

 leading to their preservation. 



But it would be superfluous to discuss this question in 



