Such a theory would not be entitled to serious consideration unless it was known that the earth was millions of years old, because we know that races of fowls separated for over three thousand years (and perhaps twice as long) and developed into quite different varieties will breed together as readily as those of the same variety. But when it is certain that the earth is so old that there has been ample time for changes in living forms that would require periods of time beyond our comprehension, some of the relations of varieties and species of birds have an important bearing on the theory of evolution. As in the case of fowls just noted, we find that domestic ducks of the same species, after a separation of several thousand years, breed freely together. But our domestic ducks are not, like the fowls, all of the same species, and if individuals of different species are paired they produce only a few weak hybrids. Our domestic geese are probably descended from two wild varieties, but races that were not brought together for thousands of years after they were domesticated are perfectly fertile together, while when mated with the American Wild Goose, which is not domesticated but will breed in captivity, they produce only hybrids. The general resemblance between geese and ducks is very striking, yet they will not breed together at all. A comparison of these facts indicates that while three thousand, or even five or six thousand, years of separation may not be enough to break down the natural affinity of varieties of the same species, separation and difference of development will eventually make of varieties distinct species, a union of which will produce only hybrids, while a longer separation and further increase of differences makes the break between the species absolute and they will not breed together at all. =Natural varieties.= A species having developed as a variety of an earlier species will continue to develop as one variety or as several varieties, according to conditions. If a part of a species becomes so separated from the rest that intercourse ceases, each division of the species may become a well-defined variety. =Varieties in domestication.= How a species when domesticated breaks up into varieties is well illustrated by the case of the fowl. The original wild species has long disappeared, but there is good reason to suppose that in size and color it was something between a Brown Pit Game and a Brown Leghorn. The birds were smaller than most fowls seen in this country to-day. The prevailing color was a dull brown, because that color best conceals a small land bird from its enemies. Fowls that were domesticated and given good care and an abundance of food would usually grow larger than the wild stock. Thus if any person, or the people generally in any community, systematically gave their fowls good care, a variety of unusual size would be developed.