rate
decreases, so does the death-rate, for a larger proportion of those born
are able to find food and to escape enemies.

When considering man, one realizes at once that relatively few babies or
adults starve to death. The selective death-rate therefore must include
only those who are unable to escape their enemies; and while these
enemies of the species, particularly certain microoerganisms, still take
a heavy toll from the race, the progress of science is likely to make it
much smaller in the future.

The different aspects of natural selection may be classified as follows:

{ Lethal { Sustentative
{ { Non-sustentative
Natural selection {
{ Reproductive { Sexual
{ { Fecundal

The lethal factor is the one which Darwin himself most emphasized.
Obviously a race will be steadily improved, if the worst stock in it is
cut off before it has a chance to reproduce, and if the best stock
survives to perpetuate its kind. "This preservation of favourable
individual differences and variations, and the destruction of those
which are injurious, I have called natural selection, or the survival of
the fittest," Darwin wrote; and he went on to show that the principal
checks on increase were overcrowding, the difficulty of obtaining food,
destruction by enemies, and the lethal effects of climate. 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 discus