we find the mammalian race slowly developing into opossums and kangaroos, such as still inhabit the isolated and antiquated continent of Australia.  Gathering strength all the time for the coming contest, increasing constantly in size of brain and keenness of intelligence, the true mammals were able at last, towards the close of the secondary ages, to enter the lists boldly against the gigantic saurians.  With the dawn of the tertiary period, the reign of the reptiles begins to wane, and the reign of the mammals to set in at last in real earnest.  In place of the ichthyosaurs we get the huge cetaceans; in place of the deinosaurs we get the mammoth and the mastodon; in place of the dominant reptile groups we get the first precursors of man himself.

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 aepyornis 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 fore limbs have been gradually modified into swimming organs, exactly analogous to the flippers of the seal.

Are the great animals now passing away and leaving no representatives of their greatness to future ages?  On land at least that is very probable.  Man, diminutive man, who, if he walked on all fours, would be no bigger than a silly sheep, and who only partially disguises his native smallness by his acquired habit of walking erect on what ought to be his hind legs—­man has upset the whole balanced economy of nature, and is everywhere expelling and exterminating before him the great herbivores, his predecessors.  He needs for his corn and his bananas the fruitful plains which were once laid down in prairie or scrubwood.  Hence it seems not unlikely that the elephant, the hippopotamus,