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.
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