186 



HYBKIDISM. CHAp< 



some one compatriot, sterility with other species would follow 

 as a necessary consequence. In the second place, it is as much 

 opposed to the theory of natural selection, as to the theory of 

 special creation, that in reciprocal crosses the male element 

 of one form should have been rendered utterly inpotent 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 reproductive system could not possibly be 

 advantageous to either species. 



In considering the probability of natural selection havin 

 come into action in rendering species mutually sterile, one 

 great difficulty will be found to lie in the existence of many 

 graduated steps from slightly lessened fertility to absolut 

 sterility. It may be admitted, on the principle above explained 

 that it would profit an incipient species if it were rendered ir 



ght degree sterile when crossed with its parent-form 





e 



with some other variety ; for thus fewer bastardised and dete- 

 riorated 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 higher degree which is common to so many 

 species, and which is universal with species which have been dif- 

 ferentiated to a generic or family rank, will find the subject extra- 

 ordinarily complex. After mature reflection it seems to me that 

 this could not have been effected through natural selection ; for 

 it could have been of no direct advantage to an individual 

 animal to breed badly with another individual of a different 

 variety, and thus leave few offspring ; consequently such indi- 

 viduals could not have been preserved or selected. Or take 

 the case of two species which in their present state, 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 



