The birds, however, even though reptile-like in their beginning, must soon have branched out on a completely new line. They for the first time among this group of animals,[195] have the perfect four-chambered heart with its quick circulation and warm blood; while not only do they use their fore limbs for flying (for this some reptiles did before them), but they use them in quite a new fashion, putting forth a clothing of feathers of wondrous beauty and construction, and with true wings taking possession of the air, where from this time their history is one of continued success. And now we have before us all the great groups of the backboned family--fish, amphibia, reptiles, birds and mammalia; but in what strange proportions! As the scenery of the Chalk Period with its fan-palms and pines comes before us, we find that the gristly fish, except the sharks and a few solitary types, are fast dying out, while the bony fish[196] are but just beginning their career. The large amphibians are all gone long ago; they have run their race, enjoyed their life and finished their course, leaving only the small newts and salamanders, and later on the frogs and toads, to keep up the traditions of the race. The land-birds are still in their earliest stage; they have probably scarcely lost their lizard-like tail, and have not yet perfected their horny beak, but are only feeling their way as conquerors of the air. And as for the milk-givers, though we have met with them in small early forms, yet now for a time we lose sight of them again altogether. It is the reptiles--the cold-blooded monster reptiles--which seem at this time to be carrying all before them. We find them everywhere--in the water, with paddles for swimming; in the air, with membranes for flying; on the land hopping or running on their hind feet. From small creatures not bigger than two feet high, to huge monsters thirty feet in height, feeding on the tops of trees which our giraffes and elephants could not reach, they fill the land; while flesh-eating reptiles, quite their match in size and strength, prey upon them as lions and tigers do upon the grass-feeders now.[197] This is no fancy picture, for in our museums, and especially in Professor Marsh’s wonderful collection in Yale Museum in America, you may see the skeletons of these large reptiles, and build them up again in imagination as they stood in those ancient days when they looked down upon the primitive birds and tiny marsupials, little dreaming that their own race, then so powerful, would dwindle away, while these were to take possession in their stead.