ON THE INFERTILITY OF CROSSES 165 
crossed with each other, and the injurious influence of close 
interbreeding is thus prevented. Had hybrids, when fairly 
treated, always gone on decreasing in fertility in each suc¬ 
cessive generation, as Gartner believed to be the case, the fact 
would have been notorious to nurserymen. 1 
Cases of Sterility of Mongrels. 
The reverse phenomenon to the fertility of hybrids, the 
sterility of mongrels or of the crosses between varieties of the 
same species, is a comparatively rare one, yet some undoubted 
cases have occurred. Gartner, who believed in the absolute 
distinctness of species and varieties, had two varieties of 
maize—one dwarf with yellow seeds, the other taller with red 
seeds; yet they never naturally crossed, and, when fertilised 
artificially, only a single head produced any seeds, and this one 
only five grains. Yet these few seeds were fertile ; so that in 
this case the first cross was almost sterile, though the hybrid 
when at length produced was fertile. In like manner, dis¬ 
similarly coloured varieties of Verbascum or mullein have been 
found by two distinct observers to be comparatively infertile. 
The two pimpernels (Anagallis arvensis and A. coerulea), classed 
by most botanists as varieties of one species, have been found, 
after repeated trials, to be perfectly sterile when crossed. 
No cases of this kind are recorded among animals; but 
this is not to be wondered at, when Ave consider how very few 
experiments have been made with natural varieties; while 
there is good reason for believing that domestic varieties are 
exceptionally fertile, partly because’one of the conditions of 
domestication was fertility under changed conditions, and also 
because long continued domestication is believed to have the 
effect of increasing fertility and eliminating whatever sterility 
may exist. This is shown by the fact that, in many cases, 
domestic animals are descended from two or more distinct 
species. This is almost certainly the case with the dog, and 
probably with the hog, the ox, and the sheep ; yet the various 
breeds are now all perfectly fertile, although we have every 
reason to suppose that there would be some degree of infer¬ 
tility if the several aboriginal species Avere crossed together 
for the first time. 
1 Origin of Species, p. 239. 
