Next above the Birds come the Mammals, which is connected with the family of Birds by several strange "connecting links"—for instance the Australian Duck-Bill, which strange creature lays eggs, and then when her eggs are hatched nourishes them with milk from her breast. In the great family of Mammals, are the following sub-families of animals, viz.: The Monotremes , or half-bird, half-mammal creatures; the Marsupials , or milk-giving, pouched animals, which carry their imperfectly developed young in an extended pouch until maturity—such as the opossum and kangaroo; the Placentals , or creatures having the placenta or appendage through which the young is nourished in the womb before birth—that is the Royal Line through which the higher forms of the Mammals proceeded.

'Of all living things there are indeed three origins only , that which springs from an egg (oviparous), that which springs from a living being...

Timaeus: they sow upon the womb, as upon ploughed soil, animalcules that are invisible for smallness and unshapen; and these, again, they mold into...