In 1831 Charles Darwin began a five-year journey as ship's naturalist on the H.M.S. The Beagle. During this time he visited South America, Australia, South Africa, and islands of the Pacific and South Atlantic. Majors in the sciences and those interested in philosophy should read this monograph. You won't find it easy reading because the language is often archaic and the arguments are sometimes difficult to follow, but it represents one of the most important contributions made to Western culture. Darwin's contribution was to provide a mechanism through which evolution could function.
Variation exists within a species.
Some of this variation has a genetic basis. Evolution can act only on traits that are passed genetically from one generation to the next. Just as an animal or plant breeder has no interest in non-genetic traits, evolution can not work on differences caused by trauma, parasitism, and other environmental variation. The reproductive potential of organisms is vast. Darwin calculated that a single pair of elephants could have 19 million descendants within 750 years if each animal lived to be 100 and each pair had six calves.