158 Experbnental Zoology 



saltations having been in most cases more than a single one, and 

 often in different directions. 



Now it is almost a universal rule — with exceptions, however — • 

 that \\'ild species are infertile when crossed with other wild species, 

 the degree of infertility differing enormously in different species 

 — ^from complete fecundity to complete sterility. It is also a 

 general rule — again with exceptions — that the more widely 

 different two species are the greater the difficulty in crossing 

 them. When the species are so different that they are put into 

 different genera, the chance of their crossing is small. If the 

 species belong to different famihes, the chance of crossing 

 is much smaller still ; and if to different orders, there is scarcely 

 any chance at all of their crossing. Crosses between the do- 

 mestic horse and zebras of different species produce infertile 

 hybrids. A jack-ass crossed with a mare (horse) gives a mule, 

 which is sterile. Conversely, a she-ass crossed with a pony gives 

 a hinny, also sterile. On the other hand, the American bison has 

 been crossed with the wild ox of Europe and has produced a 

 fertile hybrid. Similarly, the humped cattle of India crossed with 

 the domesticated ox produces fertile offspring. Crosses between 

 the common goose and the Chinese goose, which are very differ- 

 ent species, give fertile hybrids. Similarly, for the common duck 

 and the pintail duck, and for different species of pheasants.^ 

 A cross between a fowl (a langshan cock) and the common guinea 

 hen has been brought about, but the hybrid is sterile.^ 



In regard to the characters of these hybrids no general state- 

 ment can be made. Sometimes the hybrids appear to be inter- 

 mediate in one or more characters ; sometimes the character of 

 one or of the other parent predominates, and in still other cases 

 the hybrid may have characters peculiar to itself. Latent char- 

 acters may also be brought to the front by hybridizing, and 

 these dormant characters seem sometimes to be characters that 

 the ancestors of the species may be supposed to have possessed. 



' The preceding cases are quoted from Ewart's "Penycuik Experiments," 

 1899. 



^ Guyer, M. F., Science, XXI, June, 1905. 



