Private Property of 



Z. p. r^^ETCALF 



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CHAPTER VI 



THE MUTATION THEORY 



The theory that new races and species originate discontinu- 

 ously and not gradually, has received its strongest support 

 from the work of the Dutch botanist, Hugo de Vries, who 

 was one of the pioneers in the recent revival of the study of 

 evolution by experimental methods. 



De Vries began studying the variation of species of plants 

 in the field, transferring these variations to his garden and 

 there subjecting them to selection. He found that garden 

 conditions, i. e., cultivation and improved nutrition, in- 

 creased variability as regards minor differences in size, 

 luxuriance and productiveness. Such variations, which Bate- 

 son calls continuous, De Vries speaks of as fluctuating. They 

 depend, he thinks, wholly upon nutrition but do not per- 

 manently affect the specific type. This is stable, like Galton's 

 polyhedron resting securely on one of its faces. Its fluctua- 

 tions due to nutrition are like the oscillations of the poly- 

 hedron. No permanent change results from them. De 

 Vries indeed appears to think that selection acting upon 

 fluctuations {i. e., upon continuous variations) may change 

 the average condition of the race, but that such changes will 

 not persist unless maintained by rigorous selection. As soon 

 as selection ceases, he thinks, the race begins a gradual return 

 to its former condition. 



De Vries supported this view both with data from the his- 

 tory of cultivated plants and with direct experiments of his 

 own. He showed for example that in the history of the 

 cultivation of the sugar beet, the unimproved race contained 

 (about sixty years ago) from 7 to 14 per cent of sugar. Vil- 

 morin after two generations of selection of the sweetest beets 

 for seed obtained beets with 21 per cent of sugar. Since then 

 the choice of individual seed beets according to sugar-content 



71 



