Chap. IX.] WHEN CEOSSED. 37 



unnatural cross. Domesticated productions, on the other 

 hand, which, as shown by the mere fact of their domesti- 

 cation, 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 I 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 

 other ; but only a single head produced any seed, and 

 this one head produced only five grains. Manipulation 

 in this case could not have been injurious, as the plants 

 have separated sexes. No one, I believe, has suspected 

 that these varieties of maize are distinct species ; and it 

 is important to notice that the hybrid plants thus 

 raised were themselves 'perfectly fertile; so that even 

 Gartner did not venture to consider the two varieties as 

 specifically distinct. 



