Chap. XIX. HYBRIDISM. 179 



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

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

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

 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 modi- 

 fied in so peculiar a manner, that they are much more fertile 

 when crossed by 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, depends 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 fer- 

 tilise the second ; but the latter is incapable, after hundreds of 

 trials, of fertilising the former. Hybrids produced from reci- 

 procal 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- 

 longing to distinct genera can rarely, and those belonging to 

 distinct families can never, be crossed. The parallelism, how- 

 ever, is far from complete; for a multitude of closely allied 

 species will not unite, or unite with extreme difficulty, whilst 

 other species, widely different from each other, can be crossed 

 with perfect facility. Nor does the difficulty depend on ordinary 



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