Chap. IX.] SUMMARY. 263 



wterility of the illegitimate unions of dimorphic and trimorphio 

 plants and of their illegitimate progeny, perhaps render it probahle 

 that some unknown bond in all cases connects the degree of fertilitj- 

 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 seems to stand in some 

 close relation to species having been exposed for long periods oi 

 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 cor- 

 respond, 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 expe- 

 riment ; 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 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 domestication 

 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 kep'; in mind, that long-continued domestication tends 

 to eliminate sterility, and is therefore little likely to induce this 

 same quality. Independently of the question of fertility, in all 

 other respects there is the closest general resemblance between 

 hybrids and mongrels, — in their variability, in their power of 

 absorbing each other by repeated crosses, and in their inheritance 

 of characters from both parent-forms. Finally, then, although we 

 are as ignorant of the precise cause of the sterility of first crosses 

 and of hybrids as we are why animals and plants removed from 

 their natural conditions become sterile, yet the facts given in this 

 chapter do not seem to ine opposed to the belief that species 

 a'jonguially existed as varieties. 



