Placental mammals: Success
So by these means, the babies of placentalmammals can remain in the uterus until, if necessary, they are so well developed that they can be fully mobile as soon as they are born. The placental breeding technique spares the young the hazardous journey outside their mother's body at a very early stage that a marsupialneonate has to undertake, and allows their mother to supply their every want during the long period they remain within her. So whales and seals can carry their unborn young even as they swim for months through freezing seas. No marsupial with air breathing neonates in a pouch could ever succeed in doing such a thing. It is possible that the placental technique of reproduction was to prove one of the crucial factors in the mammals' ultimate success in colonising the whole of the earth.