Ckap. IX. Sterility of Hybrids. 247 



systematic affinity, by which term every kind of resemblance and 

 > dissimilarity between organic beings is attempted to be expressed. 

 The facts by no means seem to indicate that the greater or lesser 

 difficulty of either grafting or crossing various species has been a 

 special endowment ; although in the case of crossing, the difficulty 

 is as important for the endurance and stability of specific forms, 

 -as in the case of grafting it is unimportant for their welfare. 



Origin and Causes of the Sterility of first Crosses and of 



Hybrids. 



At one time it appeared to me probable, as it has to others, that 

 the sterility of first crosses and of hybrids might have been slowly 

 acquired through the natural selection of slightly lessened degrees 

 of fertility, which, like any other variation, spontaneously appeared 

 in certain individuals of one variety when crossed with those of 

 another variety. For it would clearly be advantageous to two 

 varieties or incipient species, if they could be kept from blending, 

 on the same principle that, when man is selecting at the same time 

 two varieties, it is necessary that he should keep them separate. 

 In the first place, it may be remarked that species inhabiting 

 •distinct regions are often sterile when crossed ; now it could clearly 

 have been of no advantage to such separated species to have been 

 rendered mutually sterile, and consequently this could not have 

 been effected through natural selection ; but 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 fdrm ; 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 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 



J 



