412 BIOLOGY AND ITS MAKERS 



ations. The direct influence of environment has 

 produced variations in unicellular organism. 

 5. Weismann adopts and extends the principle of 

 natural selection. Germinal selection is exhibited 

 in the germ-plasm. 

 IV. De Vries's Theory of Mutations. 



1. The formation of species is due not to gradual 



changes, but to sudden mutations. 



2. Natural selection presides over and improves varia- 



tions arising from mutation. 



From extended observations on the variability and the 

 adaptations of animals and plants, from the results of experi- 

 mental study and from intensive analysis of the various fac- 

 tors proposed to explain the process of species-forming, there 

 has resulted a remodeling of all evolutionary theories. New 

 theories have been advanced which, in their relation to Dar- 

 win's hypothesis of natural selection, fall into two categories. 

 There are competing theories designed to replace that of 

 natural selection; and there are auxiliary, or supporting 

 theories, that are designed to throw new light on the condi- 

 tions of species-forming and to strengthen the natural selec- 

 tion theory by its more complete elucidation. Such an ex- 

 tensive literature has grown up in the discussion of these 

 matters that, to cover it with any show of adequacy, re- 

 quires separate treatment, with specific illustrations and 

 extended comment. The entire case has been presented with 

 remarkable clearness in Kellogg's Darwinism To-day, and 

 since summaries of the arguments would be beyond the 

 purpose of this book, the reader is referred to Kellogg's 

 volume. 



There are, however, two ideas of such fundamental im- 

 portance in the post-Darwinism discussions that they should 



