292 G^ USES OF THE STERILITY 



abled 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 fertiUtyto ab- 

 solute 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 selec- 

 tion 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 extraordinarily complex. After mature reflec- 

 tion, it seems to me that this could not have been effected 

 through natural selection. Take the case of any two spe- 

 cies which, when crossed, produced few and sterile off- 

 spring; 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 se- 

 lection 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 be- 

 lieve that modifications in their structure and fertility 

 have been slowly accumulated by natural selection, from 

 an advantage having been thus indirectly given to the com- 

 munity 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 preser- 

 vation. 



But it would be superfluous to discuss this question in 

 detail: for with plants we have conclusive evidence that 



