_Fifthly._--All the domestic races pair readily together, and, what is equally important, their mongrel offspring are perfectly fertile. To ascertain this fact I made many experiments, which are given in the note below; and recently Mr. Tegetmeier has made similar experiments with the same result.[334] The accurate Neumeister[335] asserts that when dovecots {193} are crossed with pigeons of any other breed, the mongrels are extremely fertile and hardy. MM. Boitard and Corbié[336] affirm, after their great experience, that with crossed pigeons the more distinct the breeds, the more productive are their mongrel offspring. I admit that the doctrine first broached by Pallas is highly probable, if not actually proved, namely, that closely allied species, which in a state of nature or when first captured would have been in some degree sterile when crossed, lose this sterility after a long course of domestication; yet when we consider the great difference between such races as pouters, carriers, runts, fantails, turbits, tumblers, &c., the fact of their perfect, or even increased, fertility when intercrossed in the most complicated manner becomes a strong argument in favour of their having all descended from a single species. This argument is rendered much stronger when we hear (I append in a note[337] {194} all the cases which I have collected) that hardly a single well-ascertained instance is known of hybrids between two true species of pigeons being fertile, _inter se_, or even when crossed with one of their pure parents.