i8o ANIMAL ECOLOGY 



of the present position of the subject. Every biologist accepts 

 the fact that evolution has taken place. The problem which 

 has not yet been really solved is the exact manner in which it 

 has happened. The existence of vast numbers of undoubted 

 and complicated adaptations in physiological, psychological, 

 and structural characters makes it reasonably certain that 

 Darwin's theory of natural selection must be essentially true, 

 however we may disagree about certain parts of it. We 

 start, therefore, by assuming that natural selection is a very 

 important factor in encouraging the spread and perpetuation 

 in the population of some of the genotypic variations which 

 are constantly arising, and the cause of which is at present 

 obscure. As we shall have occasion to point out that natural 

 selection entirely fails to explain a number of phenomena in 

 nature, it is well to be absolutely clear about the matter right 

 at the start. The writer assumes that natural selection is an 

 important factor in evolution, while at the same time holding 

 that there are other agencies also at work, the nature of which 

 will be best discovered by field ecological work on animals (just 

 as Darwin and Wallace both discovered the existence of natural 

 selection after an extensive experience of field work on animals). 

 It should be further stated that the writer does not believe 

 that there is as yet any conclusive evidence in favour of " the 

 inheritance of acquired characters." 



2. The ordinary hypothesis of evolution by natural selection 

 may be summed up conveniently as follows : 



