260 HYBKIDISM, Chap. VIII. 



degree of resemblance to each other. This latter 

 statement is clearly proved by reciprocal crosses 

 between the same two species, for according as the 

 one species or the other is used as the father or 

 the mother, there is generally some difference, and 

 occasionally the widest possible difference, in the 

 facility of effecting an union. The hybrids, more- 

 over, produced from reciprocal crosses often differ in 

 fertility. 



Now do these complex and singular rules indicate 

 that species have been endowed with sterility simply 

 to prevent their becoming confounded in nature? I 

 think not. For why should the sterility be so extremely 

 different in degree, when various species are crossed, all 

 of which we must suppose it would be equally important 

 to keep from blending together? Why should the 

 degree of sterility be innately variable in the individuals 

 of the same species ? Why should some species cross 

 with facility, and yet produce very sterile hybrids ; 

 and other species cross with extreme difficulty, and yet 

 produce fairly fertile hybrids ? Why should there often 

 be so great a difference in the result of a reciprocal 

 cross between the same two species? Why, it may 

 even be asked, has the production of hybrids been per- 

 mitted ? to grant to species the special power of pro- 

 ducing hybrids, and then to stop their further propaga- 

 tion by different degrees of sterility, not strictly related 

 to the facility of the first union between their parents, 

 seems to be a strange arrangement. 



The foregoing rules and facts, on the other hand, ap- 

 pear to me clearly to indicate that the sterility both of 

 first crosses and of hybrids is simply incidental or de- 

 pendent on unknown differences, chiefly in the repro- 

 ductive systems, of the species which are crossed. The 

 differences being of so peculiar and limited a nature, 



