t 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 less 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 moment in the history of the earth_ to which their adaptations
were appropriate