244 LAWS GOVERNING THE STERILITY Chap. VIIL 



becoming confounded in nature ? I think not. For why 

 should the sterility be so extremely difTerent in degree, Avhen 

 various species are crossed, all of Avhicli we must suppose it 

 would l)e equally important to keej") from blending together? 

 Why should the degree of sterility lie innately variable in the 

 individuals of the same species ? Why should some species 

 cross with facility, and yet produce very sterile hybrids ; and 

 odier species cross with extreme dilliculty, and yet produce 

 fairly fertile hybrids ? Why should there often be so great a 

 diflerence 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 

 l)ro]iagation by dilferent 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 simplv 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 ster- 

 ility 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 so unimportant for its Avelfare in a 

 state of nature, I prcsmne that no one will suppc^se that this 

 capacity is a specialli/ 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 na- 

 ture of their sap, etc. ; but in a multitj^de of cases we can as- 

 sign no reason whatever. Great diAersity in the size of two 

 plants, one being Avoody and the other herbaceous, one being 

 evergreen and the other deciduous, and adaptation to widely- 

 different climates, do not alwaA'S prevent the two grafting to- 

 gether. As in hybridization, so with grafting, the capacity is 

 limited by systematic alhniU', for no one has been able to graft 

 trees together belonging to ((uite distinct families: and, on the 

 other hand, closely-allied species, and varieties of the same 



