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



that, if a species was rendered sterile with some one com- 

 patriot, 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 ad- 

 vantageous 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 incipienL speGies, if it were rendered 

 in some slight degree sterile when crossed with its 



-parent form or with some other variety; for thus fewer 

 "ibastardrsed'^and deteriorated ofEspring would be pro- 

 duced to commingle their blood with the new species in 

 process^F Rrmation. But he who will take the trouble 



"lo 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 

 the survival of those individuals which happened to 



