. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. 6o Biology of the Vertebrates. At first the young vestpocketed marsupials are unable to exercise the necessary muscular effort involved in sucking, and are securely attached in a passive way to the nipple by means of a sphincter-like mouth (Fig. 54), while the female expresses milk from her mammary glands down the throat of the helpless fetus by the con- traction of the abdominal muscles. Later on, as develop- ment advances, the young marsupial draws its m
. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. 6o Biology of the Vertebrates. At first the young vestpocketed marsupials are unable to exercise the necessary muscular effort involved in sucking, and are securely attached in a passive way to the nipple by means of a sphincter-like mouth (Fig. 54), while the female expresses milk from her mammary glands down the throat of the helpless fetus by the con- traction of the abdominal muscles. Later on, as develop- ment advances, the young marsupial draws its milk in the orthodox way. Osborn, in The Age of Mammals, catalogues 76 genera of marsupials of which 37 are extinct. The living ones, excepting the opossums Didelphys of North, Cen- tral, and South America, and Caenolestes of Central America, are confined to the Australian region. Extinct Eocene genera ranged over what is now Europe, as well as both Americas and Australasia. It is considered probable that the origin and spread of marsupials occurred before the ancient land bridge that joined Australia to South America had disappeared. Those forms which became isolated at that time in the marsupial Ark of Australia were afterwards able to con- tinue their handicapped existence with comparative success, since they were not brought into competi- tion with the true mammals that developed later on the other great continental areas. Man's introduction of cats, dogs, and especially rabbits into Australia has probably doomed many, if not all, species of monotremes and marsupials of that continent to extinction. As in America, a few species may be able to survive. It is a striking fact that not only all the native mammals of Australia were monotremes and marsupials, but also that the latter became diversified in much the same way as true mammals into different types adapted to various habitats. The "koala" is a bear-like form. Species resembling wolves, hyenas, cats, rabbits, j
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