124 EVOLUTION 



bility, it will tend to re-establish itself, but 

 if the range is overpassed, it will topple over 

 into a new position; also that both of them 

 are more likely to topple over towards the 

 position of primary stability than away 

 from it." 



The Mutation Theory. — In 1900 Pro- 

 fessor Hugo de Vries of Amsterdam published 

 under the title "The Mutation Theory" an 

 account of his very interesting and important 

 experiments and observations on the origin 

 of species in the vegetable kingdom. The 

 most striking of his conclusions was that 

 species arise from one another by discontinu- 

 ous leaps and bounds, as opposed to a con- 

 tinuous process. Whereas Darwin relied on 

 the action of selection on minute individual 

 variations or fluctuations, De Vries believes 

 that these have nothing to do with the origin 

 of species, which appear "all at once" by 

 mutations. Let us quote some of his char- 

 acteristic statements. 



"By the mutation theory I mean the 

 proposition that the attributes of organisms 

 consist of distinct, separate and independent 

 units. These units can be associated in 

 groups, and we find, in allied species, the same 

 units and groups of units. Transitions, such 

 as we so frequently meet with in the external 

 form both of animals and plants, are as com- 



