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

BOOK 2

higher mammals that are familiar to everybody, for they bring forth their young alive, but they bring them forth in a very helpless and undeveloped condition, and then carry them in pouches on their bellies, where they remain glued to the mother’s teats until they

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1 Le fig. 70. An Ungulate. are able to fend for themselves. They are viviparous, but only just so. An idea

of the helplessness of a young marsupial at birth may be gained from the fact that a newly-born kangaroo—of one of the biggest kinds of kangaroo—is about the size of one’s little finger. Living marsupials are found only in Australia, New Guinea and the adjacent islands—except for the opossums, which occur in America. The best-known marsupials are the kangaroos and opossums, but there are many other forms, showing a great range of specialized structure and habit. ‘There are wolf-like, cat-like, mole-like, and ant-eater-like marsupials ; and some of them even have parachutes of skin connecting the fore and hind limbs together so that they can plane through the air like a flying squirrel. In fact, the marsupial orders and sub-orders parallel most of the orders of the next section in their diversity of structure and habit.

The third section, the placental mammals, includes all the better known mammals alive to-day. They are distinguished from the marsupials by a number of points—their brains, for example, are better developed—but the essential difference lies in the way their young are produced. The placentals are thorough-going mammals ; as their name implies, they possess true placentas

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THE SCIENCE OF LIFE

GHAPTER 1

such as we have already described in the last chapter of Book 1, and for this reason they are able to house and nourish the embryo in the uterus until it is pretty nearly able to run about by itself. The marsupials, on the other hand, develop no true placenta and so bring forth their young in a state of inconvenient immaturity ; hence the need for the pouch. We need not examine the great variety of placentals in any detail (there are fifteen orders living to-day and many extinct ones); we will merely take note of some of the most interesting and important forms and refer the reader to the Zoological Gardens and the Museum of Natural History, where their characteristics can be far more profitably studied.

The placental mammals are conveniently divided into four great cohorts based upon their way of life, and particularly upon the uses to which they put their hands, feet and teeth. We may distinguish as a first cohort the great assembly known as Ungulates or hoofed mammals, a group of vegetarians having flattish-topped but knobbly or ridged molar teeth to grind up plant tissues; for the most part large beasts using their four limbs exclusively for supporting their bodies and for locomotion. We may note as examples the three largest and most important orders—the Perissodactyla (the horses and

their close relatives the asses and zebras,

thé tapirs and the rhinoceroses), which put most of their weight on the middle toe of each foot, the Artiodactyla (the pigs and hippopotami and the great cud-chewing assembly of camels, llamas, oxen, sheep, goats, antelopes, deer, and giraffes), which

Fig. 71.

An Unguiculate.

divide their weight equally between the third and fourth toes, and the Proboscidea or elephants.

The second cohort is the Unguiculates or clawed mammals. To this cohort belong, amongst other orders, the Rodentia or gnawing mammals (rabbits, rats, mice, squirrels,