Species and Varieties, Their Origin by Mutation | Page 5

new varieties, as a basis on which to build an
explanation of the processes of nature. In my opinion Darwin was quite
right, and he has succeeded in giving the desired proof. But the basis
was a frail one, and would not stand too close an examination. Of this
Darwin was always well aware. He has been prudent to the utmost,
leaving many points undecided, and among them especially the range
of validity of his several arguments. Unfortunately this prudence has
not been adopted by his followers. Without sufficient warrant they have
laid stress on one phase of the problem, quite overlooking the others.
Wallace has even gone so far in his zeal and ardent veneration for
Darwin, as to describe as Darwinism some things, which in my opinion,
had never been a part of Darwin's conceptions.
The experience of the breeders was quite inadequate to the use which
Darwin made of it. It was neither scientific, nor critically accurate.
Laws of variation were barely conjectured; the different types of
variability were only imperfectly distinguished. The breeders'
conception was fairly sufficient for practical purposes, but science
needed a clear understanding of the [6] factors in the general process of
variation. Repeatedly Darwin tried to formulate these causes, but the
evidence available did not meet his requirements.
Quetelet's law of variation had not yet been published. Mendel's claim
of hereditary units for the explanation of certain laws of hybrids
discovered by him, was not yet made. The clear distinction between
spontaneous and sudden changes, as compared with the ever-present
fluctuating variations, is only of late coming into recognition by
agriculturists. Innumerable minor points which go to elucidate the
breeders' experience, and with which we are now quite familiar, were
unknown in Darwin's time. No wonder that he made mistakes, and laid
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 in another sense, as "fluctuations." Darwin recognized both lines
of evolution; Wallace disregarded the sudden changes and proposed
fluctuations [8] as the exclusive factor. Of late, however, this point of
view has been abandoned by many investigators, especially in America.
The actual occurrence of mutations is recognized, and the battle rages
about the question, as to whether they are be regarded as the principal
means of evolution, or whether slow and gradual changes have not also
played a large and important part.
The defenders of the theory of evolution by slow accumulation of slight
fluctuations are divided into two camps. One group is called the
Neo-Lamarckians; they assume a direct modifying agency of the
environment, producing a corresponding and useful change in the
organization. The other group call themselves Darwinians or
selectionists, but to my mind with no other right beyond the arbitrary
restriction of the Darwinian principles by Wallace. They assume
fluctuating variations in all directions and leave the choice between
them to the sieve of natural selection.

Of course we are far from a decision between these views, on the sole
ground of the facts as known at present. Mutations under observation
are as yet very rare; enough