Primitive but formidable
tigers, wolves, and hyenas were multiplying, and a great
selective struggle set in. Some groups shrank from the battle by
burrowing underground like the rabbit; some, like the squirrel or
the ape, took refuge in the trees; some, like the whale and seal,
returned to the water; some shrank into armour, like the
armadillo, or behind fences of spines, like the hedgehog; some,
like the bat, escaped into the air. Social life also was probably
developed at this time, and the great herds had their sentinels
and leaders. But the most useful qualities of the large
vegetarians, which lived on grass and leaf, were acuteness of
perception to see the danger, and speed of limb to escape it. In
other words, increase of brain and sense-power and increase of
speed were the primary requisites. The clumsy early Condylarthra
failed to meet the tests, and perished; the other branches of the
race were more plastic, and, under the pressure of a formidable
enemy, were gradually moulded into the horse, the deer, the ox,
the antelope, and the elephant.
We can follow the evolution of our mammals of this branch most
easily by studying the modification of the feet and limbs.
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