However, even if we consider only the mammalia, it is impossible to accept Flourens’ law, without considerable reserve. Weismann[30] has referred to the case of the horse, which is completely adult at 4, but lives not merely 5 times that period, but 10 or even 12 times. Mice grow extremely quickly, so that they are able to reproduce at the age of 4 months. Even if we take 6 months as their period of growth, their longevity of 5 years is twice as long as it would be according to the rule of Flourens. Amongst domesticated animals, the sheep is slow in reaching maturity; it does not acquire its adult set of teeth until it is 5 years old, and cannot be regarded as adult until then. None the less, at the age of 8 or 10 years, it loses its teeth and begins to grow old, whilst by 14 it is quite senile.[31] The longevity of the sheep, therefore, is not quite three times its period of growth. If we turn to other vertebrates, the variations in the relation of growth and the duration of life are still greater. Parrots, for instance, the longevity of which is extremely great, grow very quickly. At the age of 2 years, they have acquired the adult plumage and are able to reproduce, whilst the smaller species are in the same condition at the age of one. Incubation, moreover, is very short, not more than 25 days, and in some species not three weeks. None the less, parrots are birds which enjoy a quite remarkable longevity. The incubation period of domestic geese is 30 days, and their period of growth is also short. However, they may reach a great age, cases of 80 years and of 100 years being on record. In contrast with these, ostriches, the incubation period of which is 42 to 49 days, and which take 3 years to become adult, have a relatively short life. H. Milne-Edwards[32] many years ago contended that there was no importance in the supposed law of relation between gestation and longevity. He sums up his criticism as follows: “Although the period of uterine life is longer in the horse, that animal does not live so long as a human being; and some birds, the incubation of which only lasts a few weeks, can live more than a century.”