THE DE VRIES THEORY OF EVOLUTION 377 



Museum of Natural History 

 has been a gradual increase in size from the most ancient to the present day horse. 



on the appearance of mutations among the plants in his gardens 

 and greenhouses. He observed that mutants usually transmitted 

 their peculiarities to their offspring. 



The theory of mutation differs from Darwin's theory of natural 

 selection, in the following facts. (1) Among individuals of a 

 species, different forms arise suddenly and independently, not 

 gradually. These forms differ widely from their parents. The 

 forms showing wide departures are mutations. In many cases these 

 mutations are inherited. (2) Mutations may take place in any 

 direction. They may or may not fit in with environment. They 

 may or may not be favorable. The main thing is that some are 

 capable of establishing themselves and some are not. (3) In gen- 

 eral, the unfit mutants are likely to be eliminated through natural 

 selection. (4) The fit mutants are likely to survive by natural 

 selection. The mutationist does not believe that natural selection 

 really starts the species. He believes that it controls the persist- 

 ence or disappearance of the mutant. The keynote to the theory 

 of mutations is that organisms must first appear with distinctive 

 qualities that are inheritable in order to start a new species. 



There is practically no disagreement among scientists concern- 

 ing evidences that organic evolution has taken place. But, there 

 is still much disagreement in determining which of the theories 

 thus far formulated most nearly fits the facts. The de Vries 

 theory of mutation is one that is generally accepted by many 

 scientists. 



WH. FITZ. AD. BIO. 25 



