70 ADAPTATION AND PROGRESS 



another. One might represent the germ-plasm by the metaphor of a long 

 creeping root-stock from which plants arise at intervals, these latter repre- 

 senting the individuals of successive generations. 1 



Heredity being thus explained, variation is held to be due to the 

 union of diverse sex cells 2 and possibly to katabolic influences 

 from the environment that somehow affect the germ-plasm. 3 



HUGO DE VRIES (1848- ) 

 Mutations 



Three quotations at the very beginning of De Vries' Species 

 and Varieties are suggestive of the relation between his work 

 and that of Lamarck and Darwin. " The origin of species is a 

 natural phenomenon," Lamarck; " The origin of species is an 

 object of inquiry," Darwin; "The origin of species is an object of 

 experimental investigation," this is the thesis of De Vries, and 

 to his observations and experiments, according to Sir Arthur 

 Thomson, the world is indebted for the establishment upon a 

 solid basis of the theory of evolution by mutation. 



A further relation between this theory and that of Darwin is 

 brought out in the closing words of the book referred to: " Mu- 

 tation explains the arrival of the fittest but natural selection the 

 survival of the fittest." That is, De Vries does not deny the 

 potency of natural selection, as some have asserted, but contends 

 that it is insufficient as a theory of biological evolution for it 

 takes no account of the origin of change. His chief contention 

 with the Darwinians is that natural selection operates to preserve 

 adaptive mutations rather than mere fluctuations. 4 The theory 

 in question is thus explained by Thomson: 



The general idea is that novel characters may suddenly appear, as it were, 

 full-fledged, with considerable perfectness from the moment of their emer- 

 gence, and without intergrades Unking them to the parents. Furthermore. 



1 Essays upon Heredity, p. 266; cf. pp. 184 f. For further explanation and illus- 

 tration, see Walter, Genetics, pp. 10-13. 2 Essays upon Heredity, pp. 269 f. 



3 Weismann laid all stress on the former but in his later writings admitted the 

 latter, and recent experiments have demonstrated the certainty of such source of 

 variation though the range seems very limited. See infra, pp. 73 f. 



4 Species and Varieties, their Origin and Mutation, Introduction. Cf. Kellogg, 

 op. cit., pp. 337 ff. 



