310 SUMMARY. 



secondly, that the crossing of forms, which have been ex- 

 posed to slightly different conditions of life, or which have 

 varied, favors the size, vigor and fertility of their offspring. 

 Tiie 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 re- 

 ciprocal 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 wh}^, in the 

 case of distinct species, the sexual elements should so gen- 

 erally 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 be- 

 tween the species which are crossed. Nor 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 resemblances 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 uni- 

 versal and perfect fertility surprisiug, when it is remem- 

 bered 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 

 domestication by the selection of mere external differences, 

 and that they have not been long exposed to uniform con- 

 ditions of life. It should also be especially kept in mind, 

 that long-continued domestication tends to eliminate ster- 

 ility, and is therefore little likely to induce this same 



