These causes
may be conveniently divided as in the above diagram, into sustentative
and non-sustentative. The sustentative factor has acquired particular
prominence in the human species, since Malthus wrote his essay on
population--that essay which both Darwin and Wallace confess was the
starting point of their discovery of natural selection.
There is a "constant tendency in all animated life to increase beyond
the nourishment prepared for it," Malthus declared. "It is
incontrovertibly true that there is no bound to the prolific plants and
animals, but what is made by their crowding and interfering with each
others' means of subsistence." His deduction is well known: that as man
tends to increase in geometrical ratio, and can not hope to increase his
food-supply more rapidly than in arithmetical ratio, the human race must
eventually face starvation, unless the birth-rate be reduced.
Darwin was much impressed by this argument and ever since his time it
has usually been the foundation for any discussion of natural selection.
Nevertheless it is partly false for all animals, as one of the authors
showed[52] some years ago, since a species which regularly eats up all
the food in sight is rare indeed; and it is of very little racial
importance in the present-day evolution of man.
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