174 THE GROWTH OF GROUPS 



Of these three propositions, the third one is the essence 

 of the theory, containing, as it does, the whole substance 

 of explanation, and it is also the crux of the theory. The 

 first two are within the sphere of knowledge, but the 

 third one is outside it. We can form a mental picture 

 of the progress of evolution. We can represent it by a 

 series of models or diagrams. We can turn to nature 

 and examine the steps by which it advances. Similarly, 

 we all know what is meant by elimination, a process 

 involving separation and rejection, which can be illus- 

 trated figuratively as well as described in words. It is 

 sometimes assumed that the theory of Natural Selection 

 consists wholly of these two propositions, and the hope 

 is held out that, by establishing them, we shall be able 

 to prove the theory, for it is indisputable that both pro- 

 positions lie within the sphere of knowledge. 



But what of the third proposition ? which in popular 

 opinion is the essence of Darwin's theory. Variation is 

 chaotic or "in all directions." Is it possible to know 

 whether it is so or not ? What does the phrase mean ? 

 It cannot be represented by figures or models, which 

 would express the conceptions of several persons. We 

 know that there may be agreement in regard to this 

 matter, but it seems to me that the agreement is merely 

 verbal. That is to say, those who are in agreement find 

 themselves in that state, because each is inclined to 

 employ the same words in order to describe his uncertain 

 idea. Even the verbal agreement is not close, for some 

 prefer to describe variation as scattering or fortuitous ; 

 terms more or less synonymous with the expression " in 

 all directions." 



The foregoing criticism depends on the assumption 



