246 CAUSES OF TIJE STEUILITY Cu\r. VIII. 



I believe that the still more complex laws governing!: the facili- 

 ty of first crosses are incidental on unknown dillerences in 

 their reproductive systems. These dilTcrences, in both cases, 

 follow to a certain extent, as might have been expected, sys- 

 tematic allinity, by Avhieh every kind of resemblance and dis- 

 similarity between organic beings is attempted to be ex- 

 jiressed. The facts by no means seem to me to indicate that 

 the greater or lesser dilFiculty of either grafting or crossing va- 

 rious species has been a special endowment ; although in the 

 Ciise of crossing, the difficulty is as important for the endur- 

 ance and stabiUty of specific forms, as in the case of grafting it 

 is unimportant for their Avelfare. 



Origin and Causes of the Ster'tUty of First Crosses and of 

 JLjbrids. 



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

 that the sterility of iirst crosses and of hybrids might have 

 been slowly acquired through the natural selection of slightly- 

 lessened degrees of fertility, which s]iontaneously appeared, 

 like any other variation, in certain indivitluals of one variety 

 "when crossed with 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 Iirst place, it may be 

 remarked that distinct regions are ofteii inhabited by groups 

 of species and by single species, which, when brought together 

 and crossed, are found to be more or less sterile ; now it could 

 clearly have been of no advantage to such sejiarated species to 

 have been rendered mutually sterile, and consequently this 

 could not have been effected through natiu-al selection ; but it 

 may perhaps be argued that, if a species were 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 be rendered utterly impotent on a 

 second form, Avhile at the same time the male element of this 

 second form is enabled freely to fertilize the Iirst form ; for this 

 peculiar state of the reproductive system could not possibly be 

 advantageous to either species. 



In considering the probability of natiual selection having 



