CAUSES OF THE STERILITY 313 



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

 time the male element of this second form is enabled freely 

 to fertilise the first form ; for this peculiar state of the repro- 

 ductive 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 diflficulty 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 bastardised 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 de- 

 gree 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 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 

 favour 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 towards 

 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 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 ad- 

 vantage or indirectly give any advantage to the other individ- 

 uals of the same variety, thus leading to their preservation. 



