ECOLOGY AND EVOLUTION 185 



more, since the correlation of structure, etc., with other 

 characters shown to be useful does not at present rest on many 

 well-proved examples, it cannot yet be assumed that most 

 specific characters are indirectly useful. Thus the role of 

 Natural Selection in the production of closely allied species, 

 so far as it is known at present, seems to be limited. This 

 statement is not to be taken as a wholesale denial of the power 

 of Natural Selection. The latter is not in question when 

 structural differences of a size likely to effect survival are 

 involved. It is only the capacity of selection to use on a large 

 scale the small differences between closely allied species that 

 is unproved." 



8. It seems probable that the process of evolution may 

 take place along these Hues : genotypic variations arise in 

 one or a few individuals in the population of any species and 

 spread by some means that is not natural selection ; this 

 process, combined with various factors which lead to the 

 isolation of different sections of the population from one 

 another, results in the establishment of varieties and species 

 which differ in comparatively trivial and unimportant 

 characters. Later on, natural selection is ultimately effective, 

 probably acting rather on populations than on individuals. 

 Some such hypothesis seems absolutely necessary to account 

 for the facts (driven home by ecological work in the field, and 

 by careful systematic work at home) that, on the one hand, 

 remarkable adaptations exist in all animals, while on the other 

 hand the differences between closely allied species are not 

 adaptive. This view is opposed to most of the current teach- 

 ing about evolution, which tends either to exalt unduly or deny 

 completely the power of natural selection, but it has the 

 advantage of fitting the facts, which is after all not a bad 

 recommendation. The most obvious question which arises 

 is how a variation can spread in any population unless it is 

 in some way favoured by natural selection. The process 

 which, as we have pointed out, must happen and be happening, 

 must be a mechanical one which allows of the spread of all 

 characters indiscriminately. Any really harmful one would 

 be wiped out soon enough by natural selection, and any really 



