The idea of biological evolution was supported in Classical times by the Greek and Roman atomists, notably Lucretius. With the dominance of Christianity came belief in the Biblical story of creation according to Genesis, with the doctrine that God had directly "Created kinds" of organisms which were immutable. Other ideas resurfaced, and in 17th century English the word evolution (from the Latin word "evolutio", meaning "unroll like a scroll") began to be used to refer to an orderly sequence of events, particularly one in which the outcome was somehow contained within it from the start.

Darwin opened his argument by pointing to the results of domestication, mostly through artificial selection (though environmental changes, such as more food and protection from predators, were also factors). Comparing domesticated and wild varieties, Darwin showed that the nineteenth-century definition of species was chiefly a matter of opinion, since the discovery of new linking forms often degraded species to varieties.

Darwin's theory of evolution is based on five key observations and inferences drawn from them, as summarized by the biologist Ernst Mayr:

1. Species have great fertility. They make more offspring than can grow to adulthood.
2. Populations remain roughly the same size, with modest fluctuations.
3. Food resources are limited, but are relatively constant most of the time. From these three observations it may be inferred that in such an environment there will be a struggle for survival among individuals.
4. In sexually reproducing species, generally no two individuals are identical. Variation is rampant.
5. Much of this variation is heritable.

From this Darwin infers: In a world of stable populations where each individual must struggle to survive, those with the "best" characteristics will be more likely to survive, and those desirable traits will be passed to their offspring; and that these advantageous characteristics are inherited by following generations, becoming dominant among the population through time. This is natural selection.

Darwin did not suggest that every variation and every character must have a selection value. However, he pointed out that, because of our ignorance of animal physiology and its relationship with the environment, it was extremely rash to set down any characters as valueless to their owners. It is even more important to notice that he did not suggest that every individual with a favorable variation must be selected, or that the selected or favored animals were better or higher, but merely that they were more adapted to their surroundings.

Darwin further infers that natural selection, if carried far enough, makes changes in a population, eventually leading to new species. He puts forward myriad observations as demonstrations of this, and also claims that the fossil record can be interpreted as supporting these observations. Darwin imagined it might be possible that all life is descended from an original species from ancient times. Modern DNA evidence is consistent with this idea.

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia articles "the Origin Of Species"