One of these—the hybrid between the purple and the yellow species of Lucerne which is known to botanists as Medicago media is, writes De Vries, “cultivated in some parts of Germany on a large scale, as it is more productive than the ordinary lucerne.” Other examples of perfectly fertile plant hybrids cited by De Vries are the crosses between Anemone magellanica and A. sylvestris, between Salix alba and Salix pentandra, between Rhododendron hirsutum and R. ferrugineum.
He gives an instance of a hybrid—Ægilops speltæformis, which, though fertile, is not so fertile as a normal species would be. It is worthy of note that Burbank of California has obtained a hybrid between the blackberry and the raspberry, which is not only fertile, but quite popular as producing a novel fruit.
Sterile Plant Hybrids
De Vries does not cite nearly so many examples of sterile hybrids, presumably because they are not so easy to find. He mentions the sterile “Gordon’s currant,” which is considered to be a hybrid between the Californian and the Missouri species. He also gives Cytisus adami as an absolutely sterile hybrid, this being a cross between two species of Labernum—the common and the purple.
In the case of animals the known hybrids are so much less numerous that we are able to furnish a list which may be taken as fairly exhaustive.
Fertile Mammalian Hybrids
Taking the mammals first, we find that, in addition to those cited by Darwin, there are several recorded cases of crosses between well-defined species which are fertile.
There is the hybrid between the brown bear and the polar bear, which is perfectly fertile. In the London Zoological Gardens there is a specimen of this hybrid, also one of this individual’s offspring by a pure polar bear.
The stoat has been crossed with the domestic ferret, a descendant of the polecat, a very distinct species; the resulting hybrids have nevertheless proved fertile.
The bull American bison produces with the domestic cow hybrids known as “cataloes,” which are fertile. The reverse cross of the domestic bull with the bison cow does not, however, succeed at all, which reminds us of what happens in the case of finch-hybrids.