blooded, and if the great land and air reptiles remained cold-blooded when they left the sea and exposed themselves to the more varying temperatures of a life on land they would be at a great disadvantage when the warm-blooded birds and mammals appeared, not only clothed in non-conducting coats of feathers, blubber, and hair, but possessing an internal mechanism for maintaining at a constant temperature the heat pro PROBABLE APPEARANCE OF ICHTHYOSAURUS duced by the oxidation of food material. The comparative immunity to change of temperature thus attained by birds and mammals would give them much greater activity than any cold-blooded creature could continuously display, and assure them of predominance in the animal kingdom. If, however, as is quite probable, the great reptiles had evolved some means of maintaining bodily heat, we must look for some other reason for their disappearance, and this was probably lack of brain. In many types the head is ridiculously small compared with the animal's great bulk-and bulk alone is a doubtful advantage, since it is conspicuous and needs a large amount of food for its support—and even where the head is large, as in the rhinoceros-like forms, the brain itself is tiny in proportion to the creature's size. Birds, and still more so mammals, are immensely superior in brain to any of the earlier evolved types of animal life. Birds, though certainly derived from a reptile stock, are not closely related to the pterodactyls. They are probably descended from the kangaroo-like group of dinosaurs which had fore-feet with five toes and hindfeet with only three. The fore-limbs may have become first swimming paddles, then organs used for beating A LONG-TAILED PTERODACTYL the air and raising the body out of the water for a short flight, and lastly the feathered wings which modern birds possess. The fossil remains of birds are not very numerous. We have some from the chalk rocks, including birds with teeth like those of reptiles, and two remarkable fossils of the earliest known bird from the rocks of the preceding epoch. These skeletons were found in the lithographic stone of Bavaria, and the impression of the feathers is clearly shown on the hardened mud of which it is composed. The archeopteryx, as the bird has been called, was about the size of a pigeon and resembled existing birds in its skull, merrythought, and legs, and it had four toes. It had teeth in both jaws, a long tail like a lizard's made up of many vertebræ, and a wing which shows three distinct fingers, each ending in a claw. It resembles a lizard-like reptile more than any known bird, but that it was a true bird is proved by its legs and feathers, and no doubt it had a long line of predecessors in the evolution from the reptile stock. In the more recent rock layers various remains are found, including a monstrous South American bird which stood twelve feet high and had a huge beak and claws. Great wingless birds resembling the African ostriches and Australian emus have left their skeletons in recent deposits in New Zealand, which still possesses a small wingless bird in the kiwi or apteryx. The dodo which the Portuguese and Dutch found in Mauritius rapidly became extinct because, having no dangerous enemies until the arrival of man, it had lost the power of flight. The great auk which, like the penguin, used its wings for swimming and not for flight, used to be common in the Shetland Isles, Iceland, and Greenland, but became extinct about seventy years ago. Birds which are either wingless or have ceased to use their wings for flight have lost the advantages bestowed on birds in general by their structure and warm blood— the activity which enables them to range the whole world in search of suitable environment. It is this very width of range which has given them, with mammals, a predominant place in the animal kingdom. CHAPTER X MAMMALS N the heyday of the great reptiles we begin to find traces of the highest group in the animal kingdom the mammals, destined in the course of evolution to produce man. Sprung probably from an early reptile stock, they appeared at a time when the animal world was dominated by the great carnivorous species, and with these enemies in the field the feeble progenitors of the mammal group-in size no bigger than a rat or mouse-could escape extermination only by their insignificance and their ability, as shown by their teeth, to thrive on any kind of diet. Yet, tiny as were the forms of the first mammals, they displayed such improvements of structure as were bound, if only the group avoided early destruction, to ensure its triumph over all competitors. Whether it was due to the pressure of the mammals, or to some unexplained cause, the great reptiles died out in the epoch of the chalk rocks the epoch when flowering plants attained predominance in the plant world—and the mammals, already of great variety and importance, succeeded to their supremacy. These highest of the backboned animals surpassed the earlier evolved groups in efficiency of organization. They had more effective lungs and heart, better organs of the higher senses, and a more complex brain, which showed enormous development in the later types which were successful in their struggle for existence. The head, as the seat of the intelligence and higher senses, is more conspicuously marked out from the neck and body in mammals than is the case in the lower backboned animals; the organs of scent and hearing are CHART OF THE VARIOUS STAGES IN THE EVOLUTION OF LIFE ON THE EARTH From "Secrets of the Hills," Craig (Harrap) more complex, and in the case of ears marked out by flaps of skin movable by special muscles a power lost by disuse in man in general, but persisting in a few individuals. Like birds, the mammals possessed the advantage over reptiles of warm blood, making them more independent of variations in temperature, and like the birds they had also non-conducting coats, not of feathers but of hair, a development shown by no group evolved before them, but characteristic of all |