Natural selection is basically a way for nature to choose what works and what doesn't work. For example, if an animal was born with webs to swim, but most of its food source can be gotten on land, then the animal will die off until it evolves to go on land. That is how evolution works with natural selection. In the past long ago, millions of years back, animals in the water needed oxygen to breath, but the water started to lose it. The land had air, and then the fish slowly grew legs, and their gills would evolve to lungs. Eventually they would be able to roam the land. Because there was so much oxygen with all of the shrubs, trees, and plants, that allowed the bodies of animals to grow big.
