The science of life : fully illustrated in tone and line and including many diagrams

BOOK 3

Competing imports were barred. Home industries flourished, but their products never came up to those resulting from a more rigorous competition.

South America, on the other hand, was open to the North in early Eocene times (V Ax), but only for a short time. This allowed representatives of some, but not all, of the early placental mammals to enter, and the connection was then broken, not to be re-established till the close of the Miocene (V G3), perhaps thirty million years later. Thus the inhabitants of South America, like those of Australia, were for long preserved from the full stress of competition; the difference lying in the fact that it was not marsupials, but early and primitive placentals, which there found an Ark.

Similar results followed in both regionsthe development of creatures elsewhere unknown, the flowering denied to primitive types in more strenuous regions.

The chief factor contributing to this local evolution of South American types was the total absence of carnivores and of true ungulates. The only flesh-eating mammals were primitive marsupial types, all now extinct. The absence of more specialized beasts of prey permitted a great number of sluggish, large, and inoffensive creatures to come into existence, such as armadillos and their allies, sloths, big rodents like the treeporcupines and vizcachas, opossums, and ant-eaters ; while the absence of those bestdeveloped of herbivorous running machines, the true ungulates, permitted other stocks more or less adequately to fill the gap thus left in life’s economy.

We have already mentioned the remarkable if slightly inferior imitation of horseevolution achieved by what we may call the pseudo-horses, the Litopterna. Among other remarkable ungulates not found elsewhere were the ponderous Toxodonts, with their peculiar incisor teeth, many of them outdoing the rhinoceros in bulk ; the equally ponderous Typotheres, with almost rodentlike chisel-edged front teeth; other great herbivores with toes retractile like a cat’s 5 the Astrapotheres, with two pairs of enormous tusks like exaggerated boar’s tusks, and very stumpy limbs and neck. Finally, we have the extraordinary Pyrotheres, which not only grew as large as elephants, but paralleled certain features of the elephant stock in their huge teeth and their projecting tusks ; they were unlike all other mammals in having the fore-arm and lower leg disproportionately short, scarcely more than half the length of the upper part of the limb,

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THE SCIENCE

OF LIFE CHAPTER 4 which must have given them a very grotesque appearance.

These were all flourishing, with many other groups such as the sloths and armadillos, when in the late Miocene (V C 3) the corridor was again opened to the north. It let in dogs and foxes, cats great and small, sabre-tooths, tigers, and bears, together with horses and deer, peccaries and llamas, mice and squirrels.

From this time on there was competition between the two sets of animals, the old and the new. The old types were not beaten at once, for many of them did not attain their maximum size or abundance until later. But the final result was decisive, and the great majority of them perished wholly from the face of the earth.

It is interesting to remember that it was perhaps the South American fossils which first turned Darwin’s thoughts to the idea of Evolution. When he was going round the world on the Beagle, and occupying himself with everything from coral islands to the problems of structural geology and from the habits of savages to the structure of extinct animals, he excavated a number of the abundant fossils found in the wide-spreading Pleistocene beds of the South American pampas.

Among the skeletons there preserved are those of such remarkable creatures as the gigantic Megatherium—bulky as an elephant —whose construction made it able to pull down the branches of trees to browse on, and the eight-foot Glyptodon, a veritable animal tank, protected by a dome-shaped cuirass of heavy bone. As soon as their structure is examined it becomes obvious that the Megatherium was a sloth adapted to ground life, and that the Glyptodon was simply a giant armadillo which could not roll up.

What struck Darwin’s imagination was the fact that, while these and other fossils belonged to the characteristically South American group of edentates, they were different from any living edentate. If the edentates had from of old been confined to South America and there had been able to evolve into many different types, some of which were extinguished to leave their bones as fossils, the facts would be understandable. Otherwise the existence of fossils similar in all general points, but dissimilar in detail to the living animals, and the fact that all are found in one region of the world and one only, become very difficult to understand.

About the same time, though quite unknown to Darwin, a precisely similar story was being unfolded in Australia. Clift