The decrease of natives following the Spanish conquest of tropical
America has long been one of the most striking events of history.
Popular historians sometimes speak as if most of the native population
had been killed off by the cruelty of the conquistadores. Surely such
talk could not proceed from those who are familiar with the action of
natural selection. It is obvious that when the Spaniard brought the
natives together, making them work in mines and assemble in churches, he
brought them under conditions especially favorable for infection by the
new diseases which he had brought. The aborigines of the New World, up
to the time the Spaniards came, had undergone no evolution whatever
against these diseases; consequently the evolution began at so rapid a
rate that in a few centuries only those who lived in out-of-the-way
places remain unscathed.
The same story is repeated, in a survey of the history of the Pacific
Islands. Even such a disease as whooping-cough carried off adults by the
hundred. Robert Louis Stevenson has left a graphic picture[63] of
natural selection at work:
"The tribe of Hapaa," he writes, "is said to have numbered some four
hundred when the small-pox came and reduced them by one-fourth.
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