The above-named three fossil forms of Bos have been ranked {83} by nearly all palæontologists as distinct species; and it would not be reasonable to change their denomination simply because they are now found to be the parents of several domesticated races. But what is of most importance for us, as showing that they deserve to be ranked as species, is that they co-existed in different parts of Europe during the same period, and yet kept distinct. Their domesticated descendants, on the other hand, if not separated, cross with the utmost freedom and become commingled. The several European breeds have so often been crossed, both intentionally and unintentionally, that, if any sterility ensued from such unions, it would certainly have been detected. As zebus inhabit a distant and much hotter region, and as they differ in so many characters from our European cattle, I have taken pains to ascertain whether the two forms are fertile when crossed. The late Lord Powis imported some zebus and crossed them with common cattle in Shropshire; and I was assured by his steward that the cross-bred animals were perfectly fertile with both parent-stocks. Mr. Blyth informs me that in India hybrids, with various proportions of either blood, are quite fertile; and this can hardly fail to be known, for in some districts[193] the two species are allowed to breed freely together. Most of the cattle which were first introduced into Tasmania were humped, so that at one time thousands of crossed animals existed there; and Mr. B. O'Neile Wilson, M.A., writes to me from Tasmania that he has never heard of any sterility having been observed. He himself formerly possessed a herd of such crossed cattle, and all were perfectly fertile; so much so, that he cannot remember even a single cow failing to calve. These several facts afford an important confirmation of the Pallasian doctrine that the descendants of species which when first domesticated would if crossed probably have been in some degree sterile, become perfectly fertile after a long course of domestication. In a future chapter we shall see that this doctrine throws much light on the difficult subject of Hybridism.