The first birds had teeth and vertebrated tails. The archeopteryx, which is the earliest toothed bird whose remains have yet been found, was about the size of a crow. It had thirty-two teeth and twenty caudal vertebrae. Two specimens of it have been found in the Jurassic slates of Bavaria. One of these fossils is in the British Museum, and the other in the Museum of Berlin. Other toothed birds have been found fossil by Dr. Mudge in the cretaceous chalk of North America. These last had short, fan tails like existing birds.
From the toothed birds developed the beaked birds—the keel-breasted birds (the group to which most existing birds belong) and the birds with unkeeled breasts, i.e., the ostrich-like birds. The ostrich-like birds are runners. They have rudimentary wings, and the keel of the breast-bone, which in the keel-breasted birds acts as a stay for the attachment of the wing muscles, is lacking. The ostrich-like birds are probably degenerate flyers, the flying apparatus having become obsolete through disuse. The feathers of birds are generally supposed to be the modified scales of reptiles.
The most brilliant offspring of the reptiles were the mammals, animals capable of a wider distribution over the face of the earth than the cold-blooded reptiles, on account of their hair and their warm blood. Cold-blooded animals of great size are able to inhabit but a small zone of the existing earth’s surface—the torrid belt. They cannot house themselves during the seasons of cold, as men can; nor escape to the tropics on the wings of the wind, as do the birds; nor bury themselves in subaqueous mud, as do the frogs, snakes, and crustaceans. During the Mesozoic period, when cold-blooded reptiles of gigantic size flourished over a wide area of the earth’s surface, the planet was far warmer than now. Animals, therefore, like the mammals (or birds), capable of maintaining a fixed temperature regardless of the thermal fluctuations of the surrounding media, are the only animals of large size and power capable of uninterrupted existence over the greater part of the surface of the existing earth. The pre-eminent life of the Cenozoic time was mammalian. But the decline and fall of the saurian power was not wholly due to the rise of the more dynamic mammals. It was in part due, no doubt, to adverse conditions of climate, and also to the fact that mammals and birds guard their eggs, and saurians do not.
The lowest of the mammals are the monotremes, animals which blend in a marvellous manner the characteristics of birds, reptiles, and mammals. Only two families of these old-fashioned creatures are left, the echidna and the duck-bill (ornithorhynchus), both of them found on or near that museum of biological antiquities, Australia. They are covered with hair and suckle their young like other mammals, but they have only the rudiments of milk glands, and they lay eggs with large yolks from a cloaca, like the reptiles and birds. The duck-bill hides its eggs in the ground, but the echidna hatches its eggs in a small external brooding pouch, periodically developed for this purpose. The young of the monotremes feed on the oily perspiration which exudes from the body of the mother. The monotremes first appear in the fossiliferous rocks of the Triassic Age.
From the monotreme-like mammals developed the marsupial mammals, animals possessing a purse-like pouch on the after part of the abdomen, in which they carry their young. The young of marsupials are born in an extremely immature state, and are carried in this pouch in order to complete their development. The young of the kangaroo, an animal as large as a man, are only about an inch in length when they are born. They are carried for nine months after their birth in the marsupium of the mother, firmly attached to the maternal nipple. The marsupials came into existence during the Jurassic Age, and during the next age, the Cretaceous, they arose to considerable power. During this latter age they were found on every continent. But they have been almost exterminated by their more powerful descendants.
From the marsupials developed the placental mammals, animals so called because their young are developed within the parental body in association with a peculiar nourishing organ called the placenta. From the herbivorous marsupials developed the almost toothless edentates, the rodents, or gnawing animals, the sirenians, the cetaceans, and the hoofed animals, or ungulates. The sirenians are fish-like animals with two flippers, and are often called sea-cows. They resemble whales in many respects, and are sometimes classed with them. They are plant-eaters exclusively, and are found grazing along the bottoms of tropical estuaries and rivers. They have tiny eyes, teeth fitted for grinding (not spike-like as in the whales), and a strong affection for their young, the mother, when pursued, often carrying her little one under her flippers. An immense sirenian, known as Steller’s manatee, was discovered on the Behring Islands, along the Kamschatka coast, in 1741. Twenty-seven years afterwards not one of them was left, all having been murdered by the Russian sailors. The sirenians are probably degenerate forms of land quadrupeds, having lost their hind-limbs and developed the fish-like shape in adapting themselves to aquatic conditions. They appear first in the Eocene Age.
Among the most interesting derivatives of the herbivorous marsupials, because the most aberrant, are the whales. They are true mammals—have warm blood, breathe the air with lungs, and suckle their young like other mammals. But, like the sirenians, they live in the surface of the waters, and have flippers and a fish-like tail and form. They differ from the sirenians, however, in being carnivorous, in having inguinal instead of pectoral milk glands, and in being structurally less like quadrupeds. They probably degenerated from land quadrupeds during the Jurassic period, and, owing to their longer residence in the waters, have become further removed from the quadrupedal type than the sirenians. Whales have two limbs, the hind-limbs having disappeared as a result of the pre-eminent development of the tail. The tails of whales and sirenians are flattened horizontally, not vertically, as in fishes.
Out of generalised forms of hoofed animals now extinct developed the odd-toed and even-toed races of existing ungulates. The original ungulates had five hoofs on each foot, and were highly generalised in their structure. From these original five-toed forms have arisen the variously hoofed and variously structured tribes of existing ungulates: the five-toed elephant, the four-toed tapir and hippopotamus, the three-toed rhinoceros, the two-toed camel, sheep, swine, deer, antelope, giraffe, and ox, and the one-toed horse and zebra.
The carnivorous branch of the placental animals came from the carnivorous branch of the marsupials. From early forms of carnivorous placentals developed the ape-like lemurs and those generalised forms of rapacious animals from which arose the insect-eaters, the bats, and the true carnivora. The seals represent a by-development from the main line of the carnivora, a third defection, and a comparatively recent one, from land faunas. Seals live at the meeting of the land and the waters rather than in or on the waters, as do the cetaceans and sirenians. They have retained their fur and their four limbs, but have almost lost their power of land locomotion by the conversion of their feet into flippers. The two front-limbs of seals are the only ones used as ordinary limbs are used. The hind-limbs in most seals stretch permanently out behind, the webbed digits spreading out fan-shaped on either side of the stumpy tail, and constituting a rowing apparatus functionally homologous with the tail of fishes and whales. According to Jordan, the fur seals and the hair seals are descended from different families of land carnivora, the former probably from the bears, and the latter from the cats.
The lemurs are of especial interest to human beings, because in them are found the first startling approximation in looks and structure to the ‘human form divine.’ The lemurs are monkey-like creatures living in trees, but differ enough from true monkeys to be often placed in an order by themselves. Their milk glands are abdominal instead of pectoral, as in the monkeys, and the second digit of each hand and foot ends in a claw. The most of them live in Madagascar. They are generally nocturnal in their habits, although some species are diurnal. They appear first in the Eocene rocks, and Haeckel thinks they may have developed from opossum-like marsupials in the late Cretaceous or early Eocene Age.