be the degree of fertility, only two, ON AN AVERAGE, of the descendants of
each pair survive; the others succumb either to enemies, or to
disadvantages 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