26 OUR DOMESTIC BIRDS 



conditions, may in time so change that they become separate 

 species ; and that this process may be repeated indefinitely, the 

 number of species constantly increasing and becoming more 

 diversified and more highly developed. 



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 impor- 

 tant 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 thou- 

 sand 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 hy- 

 brids. Our domestic geese are probably descended from two 

 wild varieties, but races that were not brought together for thou- 

 sands 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 pro- 

 duce 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 thou- 

 sand, 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. 



