stress on modes of descent, which have since been proved to be of minor
importance or even of doubtful validity.

Notwithstanding all these apparently unsurmountable difficulties, Darwin
discovered the great principle which rules the evolution of organisms.
It is the principle of natural selection. It is the sifting out of all
organisms of minor worth through the struggle for life. It is only a
sieve, and not a force of nature, not a direct cause of improvement, as
many of Darwin's adversaries, and unfortunately many of his followers
also, have so often asserted.

It is [7] only a sieve, which decides what is to live, and what is to
die. But evolutionary lines are of great length, and the evolution of a
flower, or of an insectivorous plant is a way with many sidepaths. It is
the sieve that keeps evolution on the main line, killing all, or nearly
all that try to go in other directions. By this means natural selection
is the one directing cause of the broad lines of evolution.

Of course, with the single steps of evolution it has nothing to do. Only
after the step has been taken, the sieve acts, eliminating the unfit.
The problem, as to the manner in which the individual steps are brought
about, is quite another side of the question.

On this point Darwin has recognized two possibilities. One means of
change lies in the sudden and spontaneous production of new forms from
the old stock. The other method is the gradual accumulation of those
always present and ever fluctuating variations which are indicated by
the common assertion that no two individuals of a given race are exactly
alike. The first changes are what we now call "mutations," the second
are designated as "individual variations," or as this term is often used