CHAP. IX.] OF FIRST CROSSES AND OF HYBRIDS. 231 



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 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 repro- 

 ductive 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 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; 



