70 Variation 
remaining types to the rank of distinct species. Polymorphic species 
may now be regarded as the link which unites ordinary variability 
with the historical production of species. But it does not appear 
that they had this significance for Darwin ; and, in fact, they exhibit 
no phenomena which could explain the processes by which one 
species has been derived from another. By thus narrowing the limits 
of the species-producing variability Darwin was led to regard 
small deviations as the source from which natural selection derives 
material upon which to act. But even these are not all of the 
same type, and Darwin was well aware of the fact. 
It should here be pointed out that in order to be selected, a 
change must first have been produced. This proposition, which 
now seems self-evident, has, however, been a source of much differ- 
ence of opinion among Darwin’s followers. The opinion that natural 
selection produces changes in useful directions has prevailed for a 
long time. In other words, it was assumed that natural selection, by 
the simple means of singling out, could induce small and useful changes 
to increase and to reach any desired degree of deviation from the 
original type. In my opinion this view was never actually held by 
Darwin. It is in contradiction with the acknowledged aim of all 
his work,—the explanation of the origin of species by means of 
natural forces and phenomena only. Natural selection acts as a 
sieve ; it does not single out the best variations, but it simply destroys 
the larger number of those which are, from some cause or another, 
unfit for their present environment. In this way it keeps the strains 
up to the required standard, and, in special circumstances, may even 
improve them. 
Returning to the variations which afford the material for the 
sieving-action of natural selection, we may distinguish two main 
kinds. It is true that the distinction between these was not clear 
at the time of Darwin, and that he was unable to draw a sharp line 
between them. Nevertheless, in many cases, he was able to separate 
them, and he often discussed the question which of the two would 
be the real source of the differentiation of species. Certain varia- 
tions constantly occur, especially such as are connected with size, 
weight, colour, etc. They are usually too small for natural selection © 
to act upon, having hardly any influence in the struggle for life: 
others are more rare, occurring only from time to time, perhaps once 
or twice in a century, perhaps even only once in a thousand years. 
Moreover, these are of another type, not simply affecting size, number 
or weight, but bringing about something new, which may be useful 
or not. Whenever the variation is useful natural selection will take 
hold of it and preserve it; in other cases the variation may either 
persist or disappear. 
