DIXEY ON MUTATIONS 93 



more heavy weight upon the theory of evolution than that 

 which is already considered by some as too great even when 

 all small variations are held to be capable of operating in 

 the production of new characters. 1 The following extract 

 from a criticism by F. A. Dixey of the writings of an upholder 

 of the mutation theory puts this admirably : " A point, how- 

 ever, on which we should like to be satisfied is this: the 

 author asserts that ' no one questions the validity of natural 

 selection as a means of exterminating types which are unfitted 

 for their environment ' ; further, he thinks it is at least pro- 

 bable that certain types have survived in consequence of 

 their ' fitness.' But, since these latter types arose, as he 

 would say, suddenly or discontinuously, how did it happen 

 that they sprang into being in such exact harmony with 

 their surroundings? Would Mr. Lock have us fall back 

 upon the theory of 'directed variation,' or, what comes to 

 the same thing, Paley's view of ' contrivance ' by special 

 creation ? If it be replied that a well-adapted type must 

 have arisen, not by one or more large mutations, but by a 

 series of mutations both numerous and minute, we should 

 wish to know how such mutations are to be distinguished 

 from continuous variations. To say, with de Vries, that 

 selection of individual differences is powerless to raise 

 permanently the mean of a species, seems perilously like 

 begging the question. As soon as the mean has been 

 permanently raised, the result would be claimed as a 

 mutation." 2 



The difficulty of reconciling adaptation with evolution 

 by mutations is perhaps rendered more evident in the cases 

 of certain parasites than in any others. In animals that are 

 not parasitic, it might be conceivable that a certain amount 

 of adaptation would be arrived at by large mutations, 

 although it requires a considerable exercise of the imagina- 



1 See p. 73, quotation from J. A. Thomson's Heredity. 



2 F. A. Dixey, "The New Evolution," Nature, vol. Ixxv., April 18, 1907 ; 

 A Review of Recent Progress in the Study of Variation, Heredity, and Evolution, 

 by R. H. Lock, London, John Murray, 1906. 



