SUMMARY 331 



The sterility of first crosses and of their hybrid progeny 

 has not been acquired through natural selection. In the 

 case of first crosses it seems to depend on several circum- 

 stances; in some instances in chief part on the early death 

 of the embryo. In the case of hybrids, it apparently depends 

 on their whole organisation 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 fertil- 

 ity of all organic beings ; and secondly, that the crossing of 

 forms, which have been exposed to slightly different condi- 

 tions 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 trimor- 

 phic plants and of their illegitimate progeny, perhaps ren- 

 der 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 modi- 

 fied, leading to their mutual infertility, we do not know; 

 but it seems to stand in some close relation to species hav- 

 ing been exposed for long periods of time to nearly uniform 

 conditions of life. 



It is not surprising that the difficulty in crossing any two 

 species, and the sterility of their hybrid offspring, should 

 in most cases correspond, even if due to distinct causes: for 

 both depend on the amount of difference between the species 

 which are crossed. Nor is it surprising that the facility of 

 effecting a first cross, and the fertility of the hybrids thus 

 produced, and the capacity of being grafted together — though 

 this latter capacity evidently depends on widely different cir- 

 cumstances — should all run, to a certain extent, parallel with 



