The history of the great birds has been somewhat more singular. Unlike the other main vertebrate classes, the birds (as if on purpose to contradict the proverb) seem never yet to have had their day. Unfortunately for them, or at least for their chance of producing colossal species, their evolution went on side by side, apparently, with that of the still more intelligent and more powerful mammals; so that wherever the mammalian type had once firmly established itself, the birds were compelled to limit their aspirations to a very modest and humble standard. Terrestrial mammals, however, cannot cross the sea; so in isolated regions such as New Zealand and Madagascar, the birds had things all their own way. In New Zealand, there are no indigenous quadrupeds at all; and there the huge moa attained to dimensions almost equalling those of the giraffe. In Madagascar, the mammalian life was small and of low grade, so the gigantic æpyornis became the very biggest of all known birds. At the same time, these big species acquired their immense size at the cost of the distinctive birdlike habit of flight. A flying moa is almost an impossible conception; even the ostriches compete practically with the zebras and antelopes rather than with the eagles, the condors, or the albatrosses. In like manner, when a pigeon found its way to Mauritius, it developed into the practically wingless dodo; while in the northern penguins, on their icy perches, the forelimbs have been gradually modified into swimming organs exactly analogous to the flippers of the seal.