The pouched marsupials of Australia and of Central and South America give birth to young that are more like embryos than true infants. These tiny immature offspring, although blind and naked, are born with the life—preserving instinct of getting from the birth opening to the mother's pouch. The marsupial pouch is like a living incubator for the infant; here it will rest and feed by attaching to one of the teats until it is mature enough to explore the outside world.

Marsupials are probably the forerunners of placental mammals, and they were widespread throughout the world in the early days of mammals. With the development of the placental mammal, competition for survival was too great and marsupials became depleted in Europe, Asia, and North America. They survived in Australia and South America because, as placental mammals steadily evolved throughout the rest of the world, these land masses became islands isolated by marine barriers. Then, as the American continents emerged, they were connected by a land bridge, and placental mammals invaded South America. Only the tough, adaptable, generic, no—frills opossum survived the placental invasion and eventually joined the modern mammals in North America.

The mode of life of marsupials is very similar to that of placental mammals and clearly illustrates parallel development. There are marsupials that are catlike, doglike, mouselike, and eaters of flesh, plants, or insects, as well as some that graze, hunt, burrow, climb trees, and even glide.

Probably the best known of the marsupials is the kangaroo, a family of about fifty—seven species. Their breeding habits have been studied thoroughly, and as early as 1629 European explorers had observed infant kangaroos in mothers' pouches. They assumed, as did the Aborigines, that the infant had been born there. It was not until 1830 that a ship's surgeon investigated a kangaroo birth and reported that the infant made its way unaided from the birth canal to the pouch. To most observers it seemed impossible for so tiny and premature a creature to make this marathon trip to the pouch with no help. Without actual observation it was logical to assume that the mother kangaroo, taking the baby in either her forepaws or lips, placed it in the pouch. Sir Richard Owen, the distinguished anatomist, argued that the newborn could have moved to the pouch only with help from the mother. He was proved wrong in 1913, when an observer described in the Perth newspaper how he had witnessed the journey of the baby from birth canal to pouch with no assistance. By 1923, when the director of the New York Zoological Gardens witnessed and described a birth, it had become accepted as fact.

In Australia the young of the kangaroo is known as a joey. Thirty—three days after mating, the female gives birth to a single infant. The newborn kangaroo is almost transparent, weighs about 0.03 ounce, is 0.8 inch long, and could easily fit into a teaspoon. Although it is blind, hairless, and poorly developed, its front limbs are quite strong and equipped with claws. With these it scrambles and claws its way up a path in the fur inadvertently cleared by the mother while cleaning up fluids released from the broken egg membrane. Within about three minutes the infant completes an incredible uphill journey of almost eight inches and climbs into the pouch. It reaches one of four nipples, which swells in its mouth so that the joey becomes attached and cannot let go.

After about a month the infant kangaroo is sufficiently developed to open its jaws. It can now move around within the pouch but does not leave the safety of the nest for several months. Its only nourishment for almost the first year will be mother's milk. The composition of the milk changes continuously to keep up with the changing needs of the developing infant.

At six to eight months the joey pokes its head outside of its living cradle and observes the outside world for the first time. Soon it is able to climb out of the mother's pouch; it hops along beside her, nibbling grass, but quickly leaps headfirst back into the pouch at the first sign or sound of danger. It continues to suckle for several months, and during this time another newborn may already be fastened to one of the other nipples inside the pouch. Incredibly, this nipple will produce milk of the true infant formula while the joey will drink milk of an entirely different composition.

Even after it is a year old, the young joey will still attempt to hide in the mother's pouch. By now, however, it is an uncomfortably tight fit for the mother. When she has had about enough, she grabs hold of the youngster's tail and flings it outside. The joey will make a number of attempts to reenter its first home, but the results will always be the same. Eventually the young kangaroo will take the hint — joey is now on its own.

From the book: 
Our Fascinating Earth