248 Causes of the Sterility Chap. ix. 



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 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, produce 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 infertility, 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 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 detail - y 

 for with plants we have conclusive evidence that the sterility of 

 crossed species must be due to some principle, quite independent of 

 natural selection. Both Gartner and Kolreuter have proved that in 

 genera including numerous species, a series can be formed from 

 species which when crossed yield fewer and fewer seeds, to species 

 which never produce a single seed, but yet are affected by the 

 pollen of certain other species, for the germen swells. It is here 

 manifestly impossible to select the more sterile individuals, which 

 have already ceased to yield seeds ; so that this acme of sterility, 

 when the germen alone is affected, cannot have been gained through 

 selection ; and from the laws governing the various grades of sterility 

 being so uniform throughout the animal and vegetable kingdoms, 

 we may infer that the cause, whatever it may be, is the same or 

 nearly the same in all cases. 



We will now look a little closer at the probable nature of the 

 differences between species which induce sterility in first crosses 

 and in hybrids. In the case of first crosses, the greater or less 

 difficulty in effecting an union and in obtaining offspring apparently 

 depends on several distinct causes. There must sometimes be a 



