HEORIES OF WEISMANN AND DE VRIES 403 



But while sexual reproduction may be evoked to explain 

 the origin of variation in higher animals, Weismann thought 

 it was not Applicable to the lower ones, and he found himself 

 driven to assume that variation in single-celled organisms is 

 owing to the direct influence of environment upon them, 

 and thus he had an awkward assumption of variations arising 

 in a different manner in the higher and in the simplest organ- 

 isms. If I correctly understand his present position, the 

 conception of variation as due to the direct influence of 

 environment is being surrendered in favor of the action of 

 germinal selection among the simplest organisms. 



Extension of the Principle of Natural Selection. — These 

 variations, once started, will be fostered by natural selection 

 provided they are of advantage to the organism in its struggle 

 for existence. It should be pointed out that Weismann is a 

 consistent Darwinian; he not only adopts the principle of 

 natural selection, but he extends the field of its operation 

 from externals to the internal parts of the germinal elements. 



"Roux and others have elaborated the idea of a struggle 

 of the parts within the organism, and of a corresponding 

 intra-selection; . . . but Weismann, after his manner, has 

 carried the selection-idea a step farther, and has pictured 

 the struggle among the determining elements of the germ- 

 cell's organization. It is at least conceivable that the stronger 

 'determinants,' i.e., the particles embodying the rudiments 

 of certain qualities, will make more of the food-supply than 

 those which are \veaker, and that a selective process will 

 ensue" (Thomson). This is the conception of germinal 

 selection. 



He has also extended the application of the general 

 doctrine of natural selection by supplying a great number 

 of new illustrations. 



The whole theory of Weismann is so well constructed 

 that it is very alluring. Each successive position is worked 



