41^ BIOLOGY AND ITS MAKERS 



ations. The direct influence of environment 

 produced variations in unicellular organism. 

 5. Weismann adopts and extends the principle o 

 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 





