Accordingly at least 60%, and considering the above mentioned
sources of error we may say two-thirds, of the child mortality is
selective in character. That accords reasonably well with the
55-74% which Pearson found for the extent of selective deaths in
his study.
In general, then, one may believe that more than a half of the persons
who die nowadays, die because they were not fit by by nature (i. e.,
heredity) to survive under the conditions into which they were born.
They are the victims of lethal natural selection, nearly always of the
non-sustentative type. As Karl Pearson says, "Every man who has lived
through a hard winter, every man who has examined a mortality table,
every man who has studied the history of nations has probably seen
natural selection at work."
There is still another graphic way of seeing natural selection at work,
by an examination of the infant mortality alone. Imagine a thousand
babies coming into the world on a given day. It is known that under
average American conditions more than one-tenth of them will die during
the first year of life. Now if those who die at this time are the
inherently weaker, then the death-rate among survivors ought to be
correspondingly less during succeeding years, for many will have been
cut down at once, who might otherwise have lingered for several years,
although doomed to die before maturity.
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