The magic of evolution

I was a kid likes to play with tadpoles, but has never been curious about why the process of growing tadpoles is so strange: the bulk of a tail slowly grow two hind legs, and then went on to grow two front legs, then to the tail disappeared color from black to green frogs, and even frogs do not need to live in the water, tadpoles version of 鲤鱼跳龙门 ah! More incredible is that why only crawl ugly caterpillar turn into colorful butterflies fly? Of course, the biology textbook told me that this is a biological metamorphosis. In vertebrates, mostly fish and amphibians such a magical function. However, as to why some biological metamorphosis, most organisms do not, it seems not entirely convincing reason to explain such strange things.
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