Chap. IX.] OF FIRST CROSSES AND OF HYBRIDS. 21 



it may perhaps be argued, that, if a species was rendered 

 sterile with some one compatriot, sterility with other 

 species would follow as a necessary contingency. In 

 the second place, it is almost as much opposed 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, 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 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 

 bastardised and deteriorated offspring would be pro- 

 duced 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 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 



