Chap. ix. Fertility of Varieties when Crossed. 257 



it becomes in the highest degree improbable that similar conditions 

 long-continued should likewise induce this tendency; though in 

 certain cases, with species having a peculiar constitution, sterility- 

 might occasionally be thus caused. Thus, as I believe, we can 

 understand why with domesticated animals varieties have not been 

 produced which are mutually sterile ; and why with plants only a 

 few such cases, immediately to be given, have been observed. 



The real difficulty in our present subject is not, as it appears to 

 me, why domestic varieties have not become mutually infertile 

 when crossed, but why this has so generally occurred with natural 

 varieties, as soon as they have been permanently modified in a 

 sufficient degree to take rank as species. We are far from precisely 

 knowing the cause ; nor is this surprising, seeing how profoundly 

 ignorant we are in regard to the normal and abnormal action of 

 the reproductive system. But we can see that species, owing to 

 their struggle for existence with numerous competitors, will have 

 been exposed during long periods of time to more uniform con- 

 ditions, than have domestic varieties ; and this may well make a 

 wide difference in the result. For we know how commonly wild 

 animals and plants, when taken from their natural conditions and 

 subjected to captivity, are rendered sterile ; and the reproductive 

 functions of organic beings which have always lived under natural 

 conditions would probably in like manner be eminently sensitive 

 to the influence of an unnatural cross. Domesticated productions, 

 on the other hand, which, as shown by the mere fact of their 

 domestication, were not originally highly sensitive to changes in 

 their conditions of life, and which can now generally resist with 

 undiminished fertility repeated changes of conditions, might be 

 expected to produce varieties, which would be little liable to have 

 their reproductive powers injuriously affected by the act of crossing 

 with other varieties which had originated in a like manner. 



I have as yet spoken as if the varieties of the same species were 

 invariably fertile when intercrossed. But it is impossible to resist 

 the evidence of the existence of a certain amount of sterility in the 

 few following cases, which 1 will briefly abstract. The evidence is 

 at least as good as that from which we believe in the sterility of a 

 multitude of species. The evidence is, also, derived from hostile 

 witnesses, who in all other cases consider fertility and sterility as 

 safe criterions of specific distinction. Gartner kept during several 

 years a dwarf kind of maize with yellow seeds, and a tall variety 

 with red seeds growing near each other in his garden ; and although 

 these plants have separated sexes, they never naturally crossed. 

 He then fertilised thirteen flowers of the one kind with pollen of the 



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