The pedigree of the human race / by Harris Hawthorne Wilder.

  • Harris Hawthorne Wilder
Date:
[1926?]
Catalogue details

Licence: In copyright

Credit: The pedigree of the human race / by Harris Hawthorne Wilder. Source: Wellcome Collection.

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    show that the marsupials were introduced into Australia very late (perhaps not earlier than the Pleistocene), and at a time when the Placental mammals were elsewhere nearly as widely distributed and as highly differentiated as at the present time. Since, now, there are no placental remains in Australia, and only late marsupial ones, it must be assumed that for some reason Australia remained for a long period free from mammals of any kind and that when the invasion did occur, it was participated in by marsupials only. (c) Placentalia. This group includes all the more com- monly known mammals, and forms much the largest division of the Class. Its fundamental characteristic, as expressed in the name, is the development of the young within the maternal uterus by means of a placenta, an organ formed by a part of the uterine wall, in combination with a portion of the fetal allantois. The teeth are fewer in number than in the more primitive of the marsupials, and exhibit a wide range of specialization, corresponding to the varied environ- ments to which they have become adapted. The carnivorous forms, like the cats, have teeth shaped for cutting and tearing; herbivorous forms, like the antelopes and horses, have teeth with flat, grinding surfaces; those that subsist on roots, nuts, and seeds, like the beaver, the squirrel, and the rat, have sharp, gnawing teeth, shaped like chisels; insect feeders, like the mole and certain bats, have teeth furnished with numerous sharp points; and those that feed mainly upon fruits and grains, with the occasional use of flesh foods, like the monkeys and apes, have a combination of flat, crushing teeth with some that are fairly well fitted for cutting and tearing. In uterine and cloacal relations the placentals are not so distinctly separated from the marsupials as seem at first sight to be the case. In some forms (e. g. aardvark, elephant, some rodents, a few bats) there are two distinct lateral uteri, which open into a common vagina by separate mouths (ora uteri); in many others the two uterine elements are
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