258 FERTILITV OF VARIETIES Chap. VIII. 



tiblc tliat similar circumstances should both induce and elimi- 

 nate the same tendency ; though in certain cases, with species 

 liaving a peculiar constitution, sterility might occasionally be 

 thus induced. Tims, as I believe, we can understand why with 

 domesticated animals varieties have not been produced which 

 arc mutually sterile ; and why with plants only a few such 

 cases, immediately to l)e given, have been observed. 



The real difficulty in our present subject is not, as it ap- 

 pears to me, why domestic varieties have not become mutually 

 infertile when crossed, but why this has so generally occun-ed 

 Avith natural varieties as soon as they have been modified in a 

 sufficient and permanent degree to take rank as species. "We 

 are far from precisely knowing the cause ; nor is this surpris- 

 ing, seeing hovv' profoundly ignorant we are in regard to the 

 normal action of the reproductive system. But we can see that 

 species, owing to their struggle for existence with numerous 

 competitors, must have been exposed to more uniform condi- 

 tions during long periods of time, than have been domestic 

 varieties ; and this may well make a wide difference in the re- 

 sult. For we know how commonly wild animals and plants, 

 when taken from their natural conditions and subjected to cap- 

 tivity, are rendered sterile ; and the reproductive functions of 

 organic beings, which have always lived and been slowly mod- 

 ilied 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 

 winch can now generally resist with undiminished fertility 

 repeated changes of conditions, might be expected to produce 

 varieties, Avhich would be little liable to have their reproduc- 

 tive 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 I will briefly abstract. 

 Tiie evidence is at least as good as that from which we believe 

 in the sterility of a multitude of s]iecies. The evidence is, also, 

 derived from hostile Avitncsses, who in all other cases consider 

 fertility and sterility as safe critej-ions of specific distinction. 

 Giirtner kept during several years a dwarf kind of maize with 

 yellow seeds, and a tall variety with red seeds growing ncai 



