4o6 BIOLOGY AXD ITS MAKERS 



5. Weismann adopts and extends the principle of 

 natural selection. Germinal selection is exhibited 

 in the germ-plasm. 

 IV. De X'ries'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. 



Among the other theories of evolution that of Eimer is 

 the most notable. He maintains that variations in organisms 

 take place not fortuitously or accidentally, but follow a per- 

 fectly determinate direction. This definitely directed evolu- 

 tion is called orthogenesis. He insists that there is con- 

 tinuous inheritance of acquired characters, and he is radically 

 opposed to the belief that natural selection plays an important 

 part in evolution. The title of his pamphlet published in 

 1898, On Orthogenesis and the Impotence oj Natural Selection 

 in S pccies-For motion, gives an indication of his position in 

 reference to natural selection. A consideration of Elmer's 

 argument would be beyond the purpose of this book. 



The cause for the general confusion in the popular mind 

 regarding any distinction between organic evolution and 

 Darwinism is not far to seek. As has been shown, Lamarck 

 launched the doctrine of organic evolution, but his views did 

 not even get a public hearing. Then, after a period of tem- 

 porary disappearance, the doctrine of evolution emerged 

 again in 1859. And this time the discussion of the general 

 theory centered around Darwin's hypothesis of natural selec- 

 tion. It is quite natural, therefore, that people should think 

 that Darwinism and organic evolution are svnonvmous terms. 

 The distinction between the general theory and any particular 

 explanation of it has, I trust, been made sufficiently clear in 

 the preceding pages. 



