CHAP. IX.] OF FIRST CROSSES AND OF HYBRIDS. 229 



the facility of effecting an union. The hybrids, moreover, pro- 

 duced 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 produc- 

 tion of hybrids been permitted 1 To grant to species the special 

 power of producing hybrids, and then to stop their further pro- 

 pagation 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 

 ui 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 differ- 

 ences, 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, <fcc. ; but in a multitude of cases we can assign no 

 reason whatever. Great diversity in the size of two plants, one 

 being woody and the other herbaceous, one being evergreen and 

 the other deciduous, and adaptation to widely different climates, 

 do not always prevent the two grafting together. As in hybridisa- 

 tion, 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 of the same species, can usually, but not invariably, 

 be grafted with ease. But this capacity, as in hybridisation, is by 





