252 THE ORIGIN OF SPECIES 



to the theory of natural selection as to that of special creation, 

 that in reciprocal crosses the male element of one form should 

 have been rendered 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 re- 

 productive 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 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 extraordinarily com- 

 plex. 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 sterihty? 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 ad- 

 vantage having been thus indirectly 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 lead- 

 ing to their preservation. 



But it Vv'ould be superfluous to discuss this question in detail: 

 for with plants we have conclusive evidence that the sterility of 

 crossed species must be due to some principle, quite independent 



