About 110 million years ago, mammals had diverged into two distinct groups, the placental mammals and the marsupial mammals. These groups evolved in two increasingly different directions with great divergence in their \_41\_ approaches. Placental mammals \_42\_ well-developed young. Since placental mammals develop inside their mother's womb for an extended period of time, their young are born quite alert and are often able to move about within just a few hours of birth. Marsupial mammals' young, \_43\_, are less-developed at birth and must crawl up the mother's abdomen to the safety of her pouch. \_44\_ inside the pouch, they continue their development until ready to move about on their own. Comparison reveals that, although placental and marsupial mammals formed separate lineages, they still evolved similar \_45\_ to the environments. In some cases, placental and marsupial mammals even physically resemble each other. The pouched marsupial mouse and the harvest mouse, the marsupial mole and the common mole, and the marsupial wombat and marmot are some examples.