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old habit of egg-laying, which is also retained to-day by the most archaic living mammals, the duckbill (Platypus) and the spiny anteater, both of Australia. These two ancient forms connect the mammals with the extinct mammal-like reptiles in several respects. The opossum, kangaroo, and other marsupials, taken as a whole, represent an early stage in the higher method of reproduction, for their young are born in a very immature state and are subsequently developed in the mother’s pouch.
Origin of the Placental Mammals
In all the higher mammals, such as the dog, cat, horse, cow, elephant, monkey, ape, and man, the internal development is carried further than in the marsupials, and a more extensive connection between the growing embryo and the womb is established by means of the after-birth or placenta. Here is another fundamental structure which man obviously owes to his mammalian predecessors.
The same is true of the breasts of the female. From the presence of rudimentary breasts in male mammals (including man) people sometimes infer that the remote ancestral forms must have been hermaphrodites, but all available evidence indicates rather that in the ancestral mammals the sexes were just as distinct as they are to-day.
The presence of breasts in the female of the human species and their ability to secrete milk for the nourishment of the young were among the facts which led the great Swedish naturalist Linnaeus, in 1758, to list mankind within the class first called by him Mammalia, and the presence of only a single pair of breasts, together with other considerations, led Linnaeus to group man as a member of his order of Primates, which included also the apes, monkeys, lemurs, and bats.
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