(c) Skull of Tasmanian wolf and skull of placental wolf (after Hardy). 2 The class mammalia has two main sub-classes*: marsupials and placentals. They have evolved, independently from each other, from a common ancestry (the now extinct therapsids, or mammal-like reptiles). The marsupial embryo is expelled from the womb in a very immature state of development and is reared in an elastic pouch attached to the mother's belly. A newborn kangaroo is a half-finished job: about an inch long, naked, blind, its hind-legs no more than embryonic buds. One might speculate whether the human infant, more developed but still helpless at birth, would be better off in a maternal pouch; one is also reminded of African or Japanese women carrying their infants strapped to their backs. But whether the marsupial method is better or worse than the placental, the point is that they differ. Pouch and placenta might be called variations in strategy within the general schema of mammalian reproduction.
