For example, the species on a single continent are more likely to be similar to one another, even if they live in vastly different environmental conditions, than species from two different continents. Darwin drew heavily upon his experience on the Beagle to suggest that ability to engage in migration was an important component of natural selection. Darwin drew upon examples of islands to help explain his ideas, using examples from his time on the Beagle. For example, he theorized that animals develop to fit certain ecological niches, and that animals of different types might fill those niches in different areas.

After discussing how geography has impacted biology, Darwin moves on to a discussion of how species are classified. He acknowledges that the science behind these classifications is imperfect, as it is based on resemblance. He states his belief that animals with similar traits share a common ancestor. In this way,...
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