CONNECTING AND MISSING LINKS
mals is shown by their complex jaw, which consisted of several bones on either side, and by their retention of a quadrate bone, as it is called, between the jaw and skull. Both of these features had been possessed by reptilian, amphibian, and fish ancestors as far back as we can trace the bony skeleton, and are thus a firmly established heritage of the race. Why the simplifying? And where shall we find the missing bones? These questions have given rise to much argument. At all events, the changes occurred concomitantly with the assumption of other diagnostic features, and in a comparatively short time. The Triassic rocks yield jaws of creatures so near the dividing line between reptile and mammal that only the most intensive modern research has decided their status as reptiles, although for years they were considered primitive mammalian forms. Mammal and bird each have warm blood, which means not only a heat-controlling mechanism, but clothing (hair or feathers) as well. Feathers are seen in Archaeornis, but fossil hair is still unknown in association with ancient mammals, so that in them warm bloodedness cannot be proved, though it may be assumed from analogy. From the mammal-like reptile sprang the reptile-like mammal, out of which true mammals in turn arose. Much of this history is recorded in hundreds of tiny jaws and teeth recovered from rocks of the reptilian age. Certain links in the chain are missing, but the main evolutionary lines are indicated by tangible evidence, not alone by inference.
Mammalian divergence has followed several lines, all traceable to a few parent stocks. These lines led to the hoofed cohort, to the clawed carnivores, and to those groups which went down to the sea and became adapted marvellously to life in the great waters—whales and sea cows of diverse lineage. Other feebler folk became the inhab-
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