Depart Indefinitely from the Original Type

Wallace explains, in his 1858 paper entitled "On the Tendency of Varieties to Depart Indefinitely from the Original Type," that wild animals are engaged in a constant struggle for food. Varieties of a single species that are better suited to their environment will have a higher probability of survival, and their numbers will increase over time; the numbers of less well suited varieties will diminish. In closing, Wallace contrasts his and Lamarck's hypotheses, using the famous giraffe example.

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The Age of Darwin