The ultimate explanation for the rule of male-male competition and female choice was seen by Darwin as anisogamy.
Darwin first defined sexual selection early in : “what I call Sexual Selection.
This depends not on a struggle for existence, but on a struggle between the males for the possession of the females; the result is not death to the unsuccessful competitor, but few or no offspring.” (Darwin 1859, p 88).
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Darwin considered sexual selection to be limited to higher animals (from arthropods on up) on two grounds; first, “it is almost certain that these animals have too imperfect senses and much too low mental powers to feel mutual rivalry, or to appreciate each other's beauty or other attractions (Darwin 1871, p 321),” and “In the lowest classes the two sexes are not rarely united in the same individual, and therefore secondary sexual characters cannot be developed” (Darwin 1871, p 321).