isadvantages
of climate, or to accident. A high degree of fertility is thus not an
indication of the special success of a species, but of the numerous
dangers that have attended its evolution. Of the six young brought forth
by a pair of elephants in the course of their lives only two survive in
a given area; similarly, of the millions of eggs which two thread-worms
leave behind them only two survive. It is thus possible to estimate the
dangers which threaten a species by its ratio of elimination, or, since
this cannot be done directly, by its fertility.

Although a great number of the descendants of each generation fall
victims to accident, among those that remain it is still the greater
or lesser fitness of the organism that determines the "selection
for breeding purposes," and it would be incomprehensible if, in this
competition, it were not ultimately, that is, on an average, the best
equipped which survive, in the sense of living long enough to reproduce.

Thus the principle of natural selection is THE SELECTION OF THE BEST FOR
REPRODUCTION, whether the "best" refers to the whole constitution,
to one or more parts of the organism, or to one or more stages of
development. Every organ, every part, every character of an animal,
fertility and intelligence included, must be improved in this manner,
and be gradually brought up in the course of generations to its highest
attainable state of perfection. And not only may improvement of parts
be brought about in this way, but new parts and organs may arise,
since, through the slow and minute steps of individual or "fluctuating"
variations, a part may be added here or dropped out there, and thus
something new is produced.

The principle of selection solved the riddle as to how what was
purposive could conceivably be brought about without the intervention
of a directing power, the riddle which animate nature presents to our
intelligence at every turn, and in face of which the mind of a Kant
could find no way out, for he regarded a solution of it as not to be
hoped for. For, even if we were to assume an evolutionary force that is
continually transforming the most primitive and the simplest forms of
life into ever higher forms, and the homogeneity of primitive times into
the infinite variety of the present, we should still be unable to infer
from this alone how each of the numberless forms adapted to particular
conditions of life should have appeared PRECISELY AT THE RIGHT MOME