Carl Sagan
All life on Earth is closely related. We have a common
organic chemistry and a common evolutionary heritage. As a result, our
biologists are profoundly limited. They study only a single kind of biology,
one lonely theme in the music of life. Is this faint and reedy tune the only
voice for thousands of light-years? Or is there a kind of cosmic fugue, with
themes and counterpoints, dissonances and harmonies, a billion different voices
playing the life music of the Galaxy?
Understanding evolution is a prerequisite in
understanding the diversity of life. There are four major forces of evolution
namely migration, mutation, genetic drift and natural selection. Among them,
natural selection is perhaps the easiest to understand because the end products
are visible
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