246 SUMMARY. [CHAP. 12. 



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 reciprocal crosses, clearly leads to the con- 

 clusion 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 seems to stand in some close 

 relation to species having 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 circumstances should all run, to a 

 certain extent, parallel with the systematic affinity of the forms 

 subjected to experiment ; for systematic affinity includes re- 

 semblances of all kinds. 



First crosses between forms known to be varieties, or sufficiently 

 alike to be considered as varieties, and their mongrel offspring> 

 are very generally, but not, as is so often stated, invariably fertile. 

 Nor is this almost universal and perfect fertility surprising, when 

 it is remembered how liable we are to argue in a circle with respect 

 to varieties in a state of nature ; and when we remember that the 

 greater number of varieties have been produced under domestica- 

 tion by the selection of mere external differences, and that they 

 have not been long exposed to uniform conditions of life. It 

 should also be especially kept in mind, that long-continued 

 f^mestication tends to eliminate sterility, and is therefore little 

 likely to induce this same quality. Independently of the question 



