Chap. IX.] SUMMARY. 45 



think that trees have been specially endowed with 

 various and somewhat analogous degrees of difficulty 

 in being grafted together in order to prevent their 

 inarching in our forests. 



The sterility of first crosses and of their hybrid 

 progeny has not been acquired through natural se- 

 lection. In the case of first crosses it seems to depend 

 on several circumstances ; in some instances in chief 

 part on the early death of the embryo. In the case of 

 hybrids, it apparently depends on their whole or- 

 ganisation having been disturbed by being compounded 

 from two distinct forms ; the sterility being closely 

 allied to that which so frequently affects pure species, 

 when exposed to new and unnatural conditions of 

 life. He who will explain these latter cases will 

 be able to explain the sterility of hybrids. This 

 view is strongly supported by a parallelism of another 

 kind : namely, that, firstly, slight changes in the 

 conditions of life add to the vigour and fertility of all 

 organic beings ; and secondly, that the crossing of 

 forms, which have been exposed to slightly different 

 conditions of life or which have varied, favours the size, 

 vigour, and fertility of their offspring. The facts given 

 on the sterility of the illegitimate unions of dimorphic 

 and trimorphic plants and of their illegitimate progeny, 

 perhaps render it probable that some unknown bond in 

 all cases connects the degree of fertility of first unions 

 with that of their offspring. The consideration of these 

 facts on dimorphism, as well as of the results of reci- 

 procal crosses, clearly leads to the conclusion that the 

 primary cause of the sterility of crossed species is 

 confined to differences in their sexual elements. But 

 why, in the case of distinct species, the sexual elements 



