380 Evolution by Jumps 



The mutation theory may be stated very simply: New 

 species arise by discontinuous variation from previous forms. 

 There is a saltation, or jump, beyond the usual fluctuation 

 of characters in the offspring of one or several individuals of 

 a species. The new collection of characters becomes estab- 

 lished because each of the new characters (or the combina- 

 tion) is inherited. 



This does not mean that the world is suddenly turned 

 topsy-turvy. It does not mean a repudiation of the old 

 aphorisms. De Vries does not claim that we can gather figs 

 from thistles or grapes from thorns. The theory means only 

 that from time to time a plant or animal bears offspring that 

 differ from the parental type in some distinct way and that 

 the new form can reproduce itself. 



Basing his conclusions upon facts drawn from the ob- 

 servations of many students of many species of plants and 

 of animals, but particularly upon his own experiments with 

 evening primroses, de Vries summarized his theory as 

 follows : 



1. The new species arise suddenly at a single step, with- 

 out transitional or intermediate forms. 



2. New species are generally fully constant from the 

 first moment of their origin. 



3. The distinctive characters of the new forms corre- 

 spond in kind and in degree with those which distinguish 

 from one another old and established species. 



4. A considerable number of new individuals of the 

 same sort usually make their appearance at the same time. 



5. Although the new types vary in a normal fashion, 

 and frequently transgress the limits dividing them from the 

 parental type, their first appearance bears no relation to 

 the normal or continuous variability of the parental species. 



6. The mutations take place indefinitely, showing no 

 special tendency to favor a particular direction. 



De Vries believed further that the tendency to give rise 

 to new mutations recurs periodically, but he had no direct 

 evidence to support this supposition. 



