In his highly readable book, One Long Argument, Ernst Mayr breaks down the body of thought often referred to as “Darwin’s Theory” into five separate and distinct theories, the second of which being “common descent.” Darwin’s second evolutionary theory (second by Mayr’s count, not Darwin’s) is really a hypothesis that could be worded this way:

All life on earth descended from a single, original, primordial form that arose eons ago.

The evidence in favor of this hypothesis is strong, but the test of the hypothesis … the means of disproving it, which is, after all, the point of stating it to begin with … is difficult to define, but like pornography to a judge, one would know it when one sees it.
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