MUTATION. 



DARWIN believed, that occasionally individuals are produced 

 which differ from their ancestors in a marked way, "sports," 

 but he does not seem to think that such sports have ever 

 played an important part in evolution. 



De Vries states as his opinion, that new species come into 

 existence, not by selection after continuous geno- variation, but 

 suddenly, as the result of abrupt changes of the "inheritable". 

 According to him, new species differ from parent-species in 

 several characters, which they have acquired with one stroke. 

 He further believes that species are on the whole stable, only 

 occasionally breaking out into periods of mutability. 



When judging the work of de Vries, it must be borne in 

 mind that these ideas were set forth just before Mendel's work 

 was rediscovered and that Genetics in these days had no con- 

 nection with Biomechanics. It was possible to believe that the 

 inherited consisted of numerous determinants, which myster- 

 iously called forth definite kinds of organs or characters in a 

 direct way. We see now, that the genes act upon the final 

 characters of an organism by influencing its development at 

 some stage or other, and we understand, that there need not be 

 any relation between the number of new genes acquired or lost 

 or changed and the importance of the changes effected in the 

 characters of the organisms in which these changes take place. 



We know, that a gene may have an important influence on 

 the development oi a given biotype, whereas the same gene 

 may not affect the development of another group of organisms 

 at all. 



Neither the periodicity of supposed mutations, nor the idea 

 that a spontaneous change of the genotype must necessarily 



