THE MUTATION THEORY 315 



pretation of the behaviour of (Enothera Lamarckiana is 

 not by any means an easy matter. 



As enunciated by de Vries, the theory of mutation 

 amounts to a very complete and definite hypothesis. 

 A large part of this author's suggestions are, however, 

 almost purely speculative, and for this reason we have 

 treated the whole at somewhat less length than it per- 

 haps deserves. Some of de Vries' speculations are, 

 indeed, more picturesque than convincing. 



Thus, de Vries regards the number of unit characters 

 each of which has arisen by a single mutation to 

 be quite limited, even in the highest organisms. 

 Three or four thousand such characters, he thinks, 

 may go to build up the hereditary endowment of 

 the most complicated species. He further supposes 

 a period of mutation to recur about once in 4,000 

 years. Four thousand multiplied by 4,000 gives 

 16,000,000 the number of years required to evolve 

 the lords of creation from a * primordial protoplasmic 

 atomic globule.' And he points out that this estimate 

 is well within the limits of geological time as allowed 

 by the physicist. In this way de Vries believes that 

 his mutation theory removes a difficulty which besets 

 the selection hypothesis the difficulty, namely, of 

 insufficient time. The selectionist may reasonably 

 reply that the amount of change necessary to produce 

 in 4,000 years, by the gradual method, a difference 

 equal to that represented by a single unit character, 

 might very well be quite imperceptible in a single 

 generation. 



We may summarize our present conclusions as to 



