1S2 THE PROBLEMS OF EVOLUTION 



However we may qualify and modify the idea 

 of mutation, we are still not free from the necessity 

 of explaining these interesting changes. Some views 

 have regarded the process as a spontaneous occur- 

 rence in the organism, which might occur periodi- 

 cally during the existence of a species or at a given 

 point in its development which has been likened 

 to the maturity of the individual. De Vries stated 

 the possible interpretations very fairly as fol- 

 lows: "If mutability is a permanent condition, it 

 has of course no beginning, and moreover is not 

 due to the agency of external circumstances." ^^ 

 "If, on the other hand, mutability is not a per- 

 manent feature, it must once have had a begin- 

 ning, and this beginning itself must have had an 

 external cause." ^^ Thanks to modern experimen- 

 tal work in genetics we may now conclude that 

 external factors may cause mutations, but the 

 sharp distinction between environment and the 

 genes is scarcely justified, for a stimulus which 

 causes mutation in one species may, even when 

 intensified, fail to produce it in another. The in- 

 herited character of the chromosomes undoubtedly 

 plays a part. In some species the genes are evi- 

 dently unstable; they change, sometimes in re- 

 sponse to conditions in the organism and sometimes 

 in response to conditions of the environment, such 



^2 Species and Varieties, Their Origin by Mutation, p. 690» 1905. 

 " Op. cii., p. 691. 



