THE LINEAGE OF MAN
To return to the fossil mammals: Our knowledge of the very long period after the mammals first appeared, during the ages when the dinosaurs and other reptiles dominated the scene, is extremely meagre. From a study of recent mammals Huxley predicted that the remote common ancestors of all the highly diversified placental mammals would be found to be small insectivorous forms, not unlike some of the recent insectivores, such as the Madagascar tenrec (Centetes) in general appearance and habits. From a study of the teeth of later mammals Professor Henry Fairfield Osborn likewise predicted that the ancestral placentals, living perhaps in the Lower Cretaceous period, would have teeth of the generalized insectivorous type. Quite recently Dr. Roy C. Andrews and his colleagues have contributed important evidence in favor of this view by discovering in a Lower Cretaceous formation in the Gobi desert in Mongolia the fossil skulls of several kinds of small mammals. Some of these combine features of the later insectivores and primitive carnivores and thus appear to afford a generalized pattern for the divergent evolution of the insectivores, carnivores, herbivores, tree-shrews, and primates, all of which are first definitely known to have lived in Palaeocene time in North America, at the beginning of the Tertiary period, or so-called Age of Mammals.
Palaeontologists are confident that these already diversified mammals were not suddenly created in Palaeocene time, holding that they were derived by evolutionary changes from the more primitive mammals of the Cretaceous and Jurassic periods, from some of which they inherited certain striking characters of their dentition.
Once we have passed out of the obscurity of the very long period during which the reptilian hosts dominate the fossil record and the mammals remain far too inconspicuous, we
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