90 HEREDITY AND VARIATION 



§ 7. De Vries on Fluctuations and Mutations. 



Professor Hugo de Vries is one of the foremost of Darwin's 

 intellectual heirs, with a rich endowment of his insight and 

 patience. Long-continued and carefully controlled observations 

 and experiments with generations of plants have led him to 

 conclusions which have given the Evolution Theory a fresh 

 start. His " Mutation Theory" is certainly one of the greatest 

 advances since Darwin's day. 



The General Idea. — ^The origin of species and varieties is an 

 object for experimental inquiry. " Comparative studies have 

 contributed all the evidence hitherto adduced for the support of 

 the Darwinian theory of descent, and given us some general ideas 

 about the main lines of the pedigree of the vegetable kingdom, 

 but the way in which one species originates from another has 

 not been adequately explained. The current belief assvimes that 

 species are slowly changed into new types. In contradiction to this 

 conception the theory of mutation assumes that new species and 

 varieties are produced from existing forms by sudden leaps. The 

 parent-type itself remains unchanged throughout this process, and 

 may repeatedly give birth to new forms. These may arise simul- 

 taneously and in groups, or separately at more or less widely distant 



periods My work claims to be in full accord with the 



principles laid down by Darwin, and to give a thorough and sharp 

 analysis of some of the ideas of variability, inheritance, selection, 

 and mutation, which were necessarily vague at his time" (From 

 preface to Species and Varieties, their Origin by Mutation" 

 Chicago and London, 1905). 



A Theoretical Implication. — De Vries's Mutation Theory 

 involves the theoretical conception that " the characters of the 

 organism are made up of elements that are sharply separated 

 from each other. These elements can be combined in groups, 

 and in related species the same combinations of elements recur. 

 Transitional forms like those that are so common in the external 



