CHAPTER XII 



ECOLOGY AND EVOLUTION 



(i, 2) Although the ordinary theory of natural selection appears, at first 

 sight, to explain almost all the phenomena produced by evolution, the 

 two greatest arguments in its favour being (3) the existence of so many 

 perfect adaptations in animals and the difficulty of imagining how 

 any but useful characters could spread in a population ; yet (4) there 

 are certain cases of colour dimorphism among animals which cannot 

 be explained on the hypothesis of natural selection. Of these one of the 

 most striking is the arctic fox, with its blue and white phases ; (5) 

 another example is the white-eared cob of the Sudan. In fact (6) it 

 seems very likely that most so-called adaptive colours in mammals are 

 not actually adaptive at all. (7) Furthermore, Richards and Robson 

 have shown that it is highly probable that very closely allied species 

 hardly ever differ in characters which are adaptive, although less closely 

 allied species may do so. (8) These lines of evidence (from field 

 observation on the ecology of the animals, and from systematics) make 

 it very probable that there must be some process in nature which 

 allows of the spread of non-adaptive characters in the population of a 

 species. (9) The nature of this process will probably be revealed by 

 ecological work on the numbers of wild animals, and (10) it is suggested 

 that one factor in the spread of non-adaptive mutations is the expansion 

 in numbers of a species after each periodic minimum in numbers, at 

 which times the struggle for existence tends to cease or to become 

 reduced. (11) Whatever may be any one's particular views on evolu- 

 tion, there is no doubt that ecological work is absolutely essential for a 

 solution of certain aspects of the problem. 



I . It may at first sight seem out of place to devote one chapter 

 of a book on ecology to the subject of evolution. The reason 

 for doing it is that the ecologist working in the field is con- 

 tinually being brought up with a sharp bump against the 

 species problem. There are, in- fact, certain aspects of the 

 problem of the origin of species which can only be successfully 

 tackled along ecological lines, and it is with these aspects that 

 we shall deal in the present chapter, although it is here only 

 possible to touch on some of the most important points. In 

 order to make quite clear what part of the evolution problem 

 is affected by ecological work, we must give a brief summary 



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