If research finds that those who drive taxicabs have enlarged areas of their hippocampi, you may be tempted to believe that driving a cab and engaging in such visuospatial activities each day causes the hippocampus areas to grow. This may or may not be the case, and this riddle demonstrates the principle of

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in 1859, charles darwin set out his theory of evolution by natural selection as an explanation for adaptation and speciation. he defined natural selection as the "principle by which each slight variation [of a trait], if useful, is preserved".

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the answer is 100% density i've answered this question before and done the quiz myself.