Whence came the mammals? That, again, is one of the questions which time alone can completely answer. But the opinion of most geologists is that they arose and developed in Asia first of all, and then spread to other continents. The rise of the mammals, which, unlike the reptiles, bring forth their young relatively mature and nourish and protect them, was contributing to the downfall of the reptiles, though it cannot be considered an actual cause. The mammals' young had a better chance of living and surviving than had the eggs of reptiles. Moreover, the mammals began with superior agility and higher brain-power. It is not surprising, therefore, that the invasion of the mammals resulted in the clumsy, affectionless, small-brained reptiles being driven either into extinction, or into the sedges and rushes, the swamps and lagoons, the coverts of the jungles, the crevices of the rocks, and the various by-ways which the mammals cared least to frequent, and that they have been kept there to this day.
At first the mammals were not very different in habit or type from one another. Small animals, which, like the shrews, moles, and hedgehogs lived on insects were among the earliest. There were others whose toes were turning to hoofs in order to fit them for fleetness; and there were some curious creatures called Coryphodons, which were like the modern tapir, though they were tusked like boars. The Coryphodon was a slow beast, with toes like those of an elephant, though it was much smaller.
In America appeared a small animal not much bigger than a fox-terrier, which was the ancestor of the horse, and of which we shall have more to say. The birds increased, and forms like those of the heron mingled in the swamps with other goose-like birds that kept in their serrated bills some traces of the teeth of their early ancestors. Others, like kingfishers, flitted over the streams; and the emu, ostrich, and moa, as well as the albatross, find their earliest representatives in the Eocene times.
It is impossible for us to follow, or even to enumerate, all the varied ancestral lines which sprang up, some of them already vigorous, in the early Tertiary times, and which developed so mightily in the successive ages. We can only trace the careers of a few such as are better and more popularly known, while admitting that there are many others equally interesting from a scientific or from any other point of view. From a geologist's point of view the most important, perhaps, of all the mammal developments was that of the elephant. The first mammal which geologists discovered that was like the elephant was the Mastodon, the American variety of which is called Tetrabelodon. But this Mastodon had no proper trunk as has the elephant. Instead of that he had a very long upper lip which apparently rested on his projecting upper tusks. Mr. Kipling once suggested that the elephant's trunk was originally formed by an accident—an unfortunate young elephant before the days of trunks having stopped to drink at a pool, and his nose being seized by a crocodile, who pulled and pulled till the nose lengthened out a trunk. There certainly was some reason for the elephant's trunk, which has developed, we do not quite know how, from a long nose. But a great deal has been found out about the early development of the elephant by Dr. Andrews of the British Museum.
Evolution of the Head, Proboscis, Nostrils, and Tusks of the Elephant
The drawings are to the same scale; the nostrils indicated by the letter N, the upper lips by L, and the tusks by T.
1. Mœritherium of Eocene Libya, with a flexible upper lip and the small incisor tusks.
2. Palæomastodon of Eocene Libya, with a short proboscis and powerful upper and lower tusks.
3. Mammoth (Elephas Columbi) from the State of Indiana, with gigantic upper tusks or ivories, and long proboscis with nostrils at the tip.
Dr. Andrews was travelling in Egypt some years ago, and joined a party of officers of the great survey of Egypt in a visit to the Great Western Desert, the rainless, sandy waste west of the Nile, not very far from what is now called the Fayoum, and where in Roman days was the great Lake Mœris, now dried up to a mere brine pool, in the salt water of which the fresh-water fishes of the Nile still live. The surveying party intended to determine the geological age of these sands, which stretch for hundreds of miles, often rising into cliffs which are cut sharp by the wind and show horizontal stratification. The geologists determined that the sands of this region were of Eocene and Miocene Age, and from them Dr. Andrews brought home some very interesting bones. These included the remains of a more primitive Mastodon than any as yet known, and of an animal which he called Meritherium, which is a connecting link between elephants and other mammals. The collection included also remains of great flesh-eating beasts, and of sea cows, of tortoises, and of a snake sixty feet long!