39^ Evolution by Jumps 



to defective eyes in the offspring; exposure of the mother 

 to radium brings about congested capillaries of the skin of 

 baby mice, and absolutely nothing corresponding to that in 

 the mother. 



• 



Mutations and Evolution 



We have already noted the various alterations that can 

 take place in the chromosome material, including the genes 

 (pages 272 ff ) . Some of these alterations represent recombi- 

 nations of factors already present in the hereditary constitu- 

 tion of the strain, and bring about the development of 

 exceptional individuals. Strictly speaking such individuals 

 would not now be considered mutants. With each novelty 

 it would be necessary to determine, by means of breeding 

 experiments, the composition of the hereditary make-up and 

 to find out whether the new phenotype represents also a 

 new genotype. It has been adequately established as a gen- 

 eral fact that true mutations do occur, and it is from these 

 that evolution takes its start in each case. Each of de Vries' 

 six major propositions regarding evolution by mutation is 

 supported by the facts; and there are no known facts that 

 contradict them. It is too soon, however, to say that this 

 is the exclusive manner of evolution; and there remain many 

 unsolved problems, even if the mutation theory should be 

 completely established. 



It has been pointed out that many new ideas meet with 

 opposition not so much because they are in themselves un- 

 reasonable or improbable, but because they conflict with a 

 prevailing mode of thinking — what we like to call " com- 

 mon sense " even where it represents only common igno- 

 rance. The theory of mutation illustrates this especially 

 well since it came into conflict not only with a certain 

 passive resistance to evolutionary thinking, but also with 

 the wide diffusion of ideas about ** evolution " as a very 

 gradual process — the very idea which Darwin stressed by 

 his frequent reference to the doctrine that Nature makes 



