Natural selection.
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   Darwin  formed  his  hypothesis  of  natural  selection by drawing two
   inferences based on his observations.
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<Li> Observation  #1 : Individuals of a population exhibit variation in
their traits.
<Li> Observation #2 : Many traits are heritable.
<Li> Observation #3 : Many species overproduce offspring.
<Li> Observation #4 : Resources are limited; many of these offspring do
not survive due to competition for those resources.
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<Li> Inference  #1  (natural  selection): Individuals whose traits give
them a high probability of surviving and reproducing are likely to
leave more offspring than others.
<Li> Inference  #2 (adaptation): A population will accumulate favorable
traits over generations and become adapted to its environment.
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