See, this is the sort of question I'd be happy to answer, except that I suspect you are trying to get someone to answer your science homework for you.
READ YOUR TEXTBOOK. It is there for a reason. Barring that, read THIS instead. Scroll down a little over halfway, to the picture of the opossum, and read what it says about placental and marsupial mammals.
http://biology.clc.uc.edu/courses/bio303…
READ YOUR TEXTBOOK. It is there for a reason. Barring that, read THIS instead. Scroll down a little over halfway, to the picture of the opossum, and read what it says about placental and marsupial mammals.
http://biology.clc.uc.edu/courses/bio303…
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is called evolution and natural selection....
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The type of environment.