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Origin of Species, by Charles Darwin
Chapter II:
Variation Under Nature
Variability — Individual Differences — Doubtful Species — Wide-ranging, much diffused, and common Species vary most — Species of the Larger Genera in each Country vary more frequently than the Species of the Smaller Genera — Many of the Species included within the Larger Genera resemble Varieties in being very closely, but unequally, related to each other, and in having restricted ranges — Summary
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