Chap. IX.] SUMMARY. 45 



difficulty in being grafted together in order to preTent 

 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 ease of 

 hybrids, it apparently depends on their whole organi- 

 sation 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, thatjLhe_jcrossing_of^ forms, which have 

 been exposed to slightly different conditions^ of life" 

 or which have varied, favours the size, vigour, and fer- 

 tility ^TtEeir offspring. , The facts given on the steril- 

 ity of the illegitimate unions of dimorphic and trimor- 

 phic 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 reciprocal 

 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 should 

 so generally have become more or less modified, leading 

 to their mutual infertility, we do not know; but it 



