As to the sterility of hybrids _inter se_, or with either of the parent forms, information is still wanted. Delage, summing up the evidence in a general way, states that mongrels are more fertile and stronger than their parents, while hybrids are at least equally hardy but less fertile. While many of the hybrid products of horticulturists are certainly infertile, others appear to be indefinitely fertile.
Focke, it is true, states that the hybrids between _Primula auricula_ and _P. hirsuta_ are fertile for many generations, but not indefinitely so; but, while this may be true for the particular case, there seems no reason to doubt that many plant hybrids are quite fertile. In the case of animals the evidence is rather against fertility. Standfuss, who has made experiments lasting over many years, and who has dealt with many genera of Lepidoptera, obtained no fertile hybrid females, although he found that hybrid males paired readily and successfully with pure-bred females of the parent races. Elliot, dealing with birds, concluded that no hybrids were fertile with one another beyond the second generation, but thought that they were fertile with members of the parent races. Wallace, on the other hand, cites from Quatrefages the case of hybrids between the moths _Bombyx cynthia_ and _B. arrindia_, which were stated to be fertile _inter se_ for eight generations. He also states that hybrids between the sheep and goat have a limited fertility _inter se_. Charles Darwin, however, had evidence that some hybrid pheasants were completely fertile, and he himself interbred the progeny of crosses between the common and Chinese geese, whilst there appears to be no doubt as to the complete fertility of the crosses between many species of ducks, J. L. Bonhote having interbred in various crosses for several generations the mallard (_Anas boschas_), the Indian spot-bill duck (_A. poecilorhyncha_), the New Zealand grey duck (_A. superciliosa_) and the pin-tail (_Dafila acuta_). Podmore's pigeon hybrids were fertile _inter se_, a specimen having been exhibited at the London Zoological Gardens. The hybrids between the brown and polar bears bred at Halle proved to be fertile, both with one of the parent species and with one another.