MUTATION 267 



believe'' in the mystic sentiments for which these terms 

 always stand. That a progressive series of advances in a 

 gene might take place with a consequent advance in the 

 many characters involved is thinkable, especially if it could 

 be shown that environmental changes cause "parallel prog- 

 ress in the gene, and this in turn on the character. How 

 probable this is the reader must decide for himself in the 

 light of the very clear evidence that each character is 

 affected by changes in many genes differently located in 

 the germ-plasm, and that it is not a progressive change in 

 one gene that makes selection possible, but changes in any 

 one of many genes. 



Chance Mutation and Natural Selection 

 The mutation process rests its argument for evolution 

 on the view that among the possible changes in the genes, 

 some combinations may happen to produce characters that 

 are better suited to some place in the external world than 

 were the original characters. Apparently this appeal to 

 chance, like Darwin's appeal, has offended some of the 

 adherents of the doctrine of organic evolution, because it 

 has seemed to them inconceivable that chance could ever 

 bring about the assembling of such an intricate piece of 

 machinery as a highly complex organism. The attempt 

 to mitigate the rude shock of the appeal to chance was 

 made by Darwin by pointing out that evolution had been 

 gradual and that the assemblage has not taken place out 

 of chaos, but each stage has been built up on one a little 

 less complex than the preceding one. Nevertheless the 

 fact remains that persistent efforts continue to be made 

 from time to time to introduce into the theory of evolution 

 some sort of directive mystical agency. The Laniarckiaii 

 theory has tried to bring about a more immediate relation 

 between the organism and its environment of such a kind 

 that the adaptive change that appears in the body as a 

 result of a reaction between the environment and the ani- 

 mal or plant, is reflected into the germ-plasm. Bergson 



