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. 



