Chap* IX. of first Crosses and of Hybrids. 245 



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 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 permitted ? To grant to species the special power 

 of producing hybrids, and then to stop their further propagation by 

 different degrees of sterility, not strictly related to the facility 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 differences 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, &c. ; but in a multitude of cases we can assign no reason what- 

 ever. Great diversity in the size of two plants, one being woody 

 and the other herbaceous, one being evergreen and the other deci- 

 duous, and adaptation to widely different climates, do not always 

 prevent the two grafting together. As in hybridisation, so with 

 grafting, the capacity is limited by systematic affinity, for no one 

 has been able to graft together trees belonging to quite distinct 

 families ; and, on the other hand, closely allied species, and varieties 



