But lethal natural selection is only half the story. It is obvious that
if the constitution of a race can be altered by excess of deaths in a
certain class, it can equally be altered by excess of births in a
certain class. This is reproductive selection, which may appear in
either one of two forms. If the individual leaves few or no progeny
because of his failure to mate at the proper time, it is called sexual
selection; if, however, he mates, yet leaves few or no progeny (as
compared with other individuals), it is called fecundal selection.
Even in man, the importance of the role of reproductive selection is
insufficiently understood; in the lower animals scientists have tended
still more to undervalue it. As a fact, no species ordinarily multiplies
in such numbers as to exhaust all the food available, despite the
teaching of Malthus and Darwin to the contrary. The rate of reproduction
is the crux of natural selection; each species normally has such a
reproduction rate as will suffice to withstand the premature deaths and
sterility of some individuals, and yet not so large as to press unduly
upon the food supply. The problem of natural selection is a problem of
the adjustment between reproductive rate and death-rate, and the
struggle for subsistence is only one of several factors.
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