EVOLUTIONAL ZOOLOGY 225 



deviations from the mean, can of necessity lead only in the 

 direction of the greater or less development of characters al- 

 ready present. They can, therefore, produce nothing new. 

 When a new character appears in an organism, it must arise 

 as a wholly new, definite change — a change of the kind that 

 de Vries designates as a mutation. His experiments have 

 shown that fluctuations and mutations have nothing in com- 

 mon, and he has brought forward convincing evidence to show 

 that only those variations are permanent acquisitions that be- 

 long to the type distinguished as mutation. Fluctuations, on 

 the other hand, do not lead to permanent modifications, since 

 they depend merely upon the fluctuating play of nutritive and 

 other environmental stimuli upon the organism. 



It is impossible for lack of time to present the evidence, 

 drawn from his observations and experiments, on which de 

 Vries has based his doctrine of the method of evolution, or his 

 so-called mutation theory. Suffice it to say, that he claims to 

 have actually observed the appearance of several new forms 

 of plants which came into existence in a single generation, 

 differing in many characters from the parent stock and also 

 from each other. These new forms, arising by mutation, he 

 terms elementary species, and he maintains that they breed 

 true to their new characters even under widely different en- 

 vironmental conditions, while the parent stock which threw 

 them off remains unchanged. The fact of the widespread 

 occurrence of mutations not only in plants but in animals as 

 well is now thoroughly established. 



It should be pointed out, however, that, even though the 

 investigations of de Vries and others have shown that, as a 

 theory of the method of species-formation, natural selection 

 must be abandoned, it must still be effective in directing the 

 general course of evolutionary development by acting as a 

 sieve through which all organisms, new or old, must pass and 

 thus eliminating all lines which are fundamentally unadaptive 

 to the conditions of the environment. "With the single steps 



