OF FIRST CROSSES AND OF HYBRIDS. 271 



ing 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, moreover, 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 sup- 

 pose 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 diffi- 

 culty, and yet produce fairly fertile hybrids ? Why should 

 there often be so great a difference in the result of a re- 

 ciprocal 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 producing 

 hybrids, and then to stop their further propagation by dif- 

 ferent degrees of sterility, not strictly related to the facil- 

 ity of the first union between their parents, seems a strange 

 arrangement. 



The foregoing rules and facts, on the other hand, appear 

 to me clearly to indicate that the sterility, both of first 

 crosses and of hybrids, is simply incidental or dependent 

 on unknown differences in their reproductive systems ; the 

 differences being of so peculiar and limited a nature, that, 

 in reciprocal crosses between the same two species, the male 

 sexual element of the one will often freely act on the female 

 sexual element of the other, but not in a reversed direction. 

 It will be advisable to explain a little more fully, by an 

 example, what I mean by sterility being incidental on other 

 differences, and not a specially endowed quality. As the 

 capacity of one plant to be grafted or budded on another is 

 unimportant for their welfare in a state of nature, I presume 

 that no one will suppose that this capacity is a specially 

 endowed quality, but will admit that it is incidental on dif- 

 ferences in the laws of growth of the two plants. We can 

 sometimes see the reason why one tree will not take on 

 another, from differences in their rate of growth, in the 

 hardness of their wood, in the period of the flow or nature 

 of their sap, etc. j but in a multitude of cases we can assign 



