320 THE VARIATION OF ANIMALS IN NATURE 



Difficult as it may be to explain the origin of such structures 

 by Natural Selection, it is far more of a strain on our credulity 

 to believe that they could be produced by chance. It will 

 be thus seen that the view that evolution may have been 

 produced solely by chance, labours under a serious general 

 disadvantage, so that any evidence that the non-advantageous 

 mutation can survive and multiply must be exceptionally 

 strong. 



With these qualifications in mind let us examine Elton's 

 theory of the multiplication of non-adaptive mutations. 

 As a preliminary it should be pointed out that he makes 

 (1930, pp. 89-90) a distinction between the origin of adaptation 

 and the origin of species. The former he attributes to Natural 

 Selection ; the latter to his special theory which we shall 

 examine immediately. It is necessary, however, to comment 

 on the antithesis just noted. Much that has appeared in the 

 past pages must seem to justify a belief that adaptation and 

 the origin of species are separate phenomena and due to 

 separate causes. We shall discuss this in the last chapter. It 

 is enough now to note that Elton does not discuss their inter- 

 relationships, nor does he question how specific differences are 

 raised to generic. He suggests (1924, p. 156, and 1930, p. 78) 

 that the spread of non-advantageous mutants might be facili- 

 tated by the periodic fluctuations of the numbers of animal 

 populations. During a period when numbers are at a mini- 

 mum as the result of wholesale destruction by epidemics, bad 

 weather, etc., there would be theoretically at least a cessation 

 of competition and an increased likelihood of the survival of 

 a given mutant. As an extension of this idea we have to point 

 out that a similar reduction of competition and lowering of the 

 death-rate are observable when a predacious or parasitic enemy 

 is reduced numerically — e.g. by an epidemic (cf. Thompson, 

 1928, and this work, p. 193). The incidence of disease is 

 known to lead to big reductions in the numbers of a natural 

 population — e.g. in Red and Grey Squirrels (Middleton, 1931 ; 

 and cf. Elton, 1931, for the effects of epidemics in general). 

 We believe that the frequency of epidemics among animals in 

 nature has been seriously underestimated. 



Elton's suggestion is open, however, to several serious ob- 

 jections which indeed he has himself considered (1930, p. 79 ; 

 see also Haldane, 1932, p. 204). 



