Chap. XIX. HYBRIDISM. 163 



identical, or nearly identical, in the animal and vegetable 

 kingdoms. 



Secondly, the sterility of distinct species when first united, 

 and that of their hybrid offspring, graduate, by an almost 

 infinite number of steps, from zero, when the ovule is never 

 impregnated and a seed-capsule is never formed, up to com- 

 plete fertility. We can only escape the conclusion that some 

 species are fully fertile when crossed, by determining to 

 designate as varieties all the forms which are quite fertile. 

 This high degree of fertility is, however, rare. Nevertheless, 

 plants, which have been exposed to unnatural conditions, 

 sometimes become modified in so peculiar a manner, that they 

 are much more fertile when crossed with a distinct species than 

 when fertilised by their own pollen. Success in effecting a 

 first union between two species, and the fertility of their 

 hybrids, depend in an eminent degree on the conditions of 

 life being favourable. The innate sterility of hybrids of the 

 same parentage and raised from the same seed-capsule often 

 differs much in degree. 



Thirdly, the degree of sterility of a first cross between two 

 species does not always run strictly parallel with that of their 

 hybrid offspring. Many cases are known of species which 

 can be crossed with ease, but yield hybrids excessively 

 sterile ; and conversely some which can be crossed with 

 great difficulty, but produce fairly fertile hybrids. This is 

 an inexplicable fact, on the view that species have been 

 specially endowed with mutual sterility in order to keep 

 them distinct. 



Fourthly, the degree of sterility often differs greatly in two 

 species when reciprocally crossed ; for the first will readily 

 fertilise the second ; but the latter is incapable, after hundreds 

 of trials, of fertilising the former. Hybrids produced from 

 reciprocal crosses between the same two species likewise 

 sometimes differ in their degree of sterility. These cases 

 also are utterly inexplicable on the view of sterility being a 

 special endowment. 



Fifthly, the degree of sterility of first crosses and of hybrids 

 runs, to a certain extent, parallel, with the general or system- 

 atic affinity of the forms which are united. For species be- 



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