CHAPTER X 



EXTENSIONS OF THE THEORY OF SELECTION AND 

 OTHER EVOLUTIONARY THEORIES 



Why there are numerous species to-day. Isolation facilitates the 

 divergence of species. Modification of isolated animals. Move- 

 ment and alteration of animals. Are species formed by isolation 

 even without the aid of natural selection? Definite variations. 

 Germinal selection. Changes in the nutrition of the embryo are 

 the foundation of variations. Do useless organs disappear through 

 germinal selection? Refutation of germinal selection. Effect 

 of external influences as modifying principle. Orthogenesis. 

 Rejection of same. The mutation theory. Variations and 

 mutations. Do variations proceed indefinitely? Is there a 

 formative energy in organisms? Mechanicism and vitalism. 

 How are the form and purposiveness of organisms explained? 

 What is to be understood by chance. Absence of purpose in 

 living things. The will to live, the instinct of self-preservation. 



We can now picture to ourselves how life began on 



our planet, and how it has been evolved. In virtue 



of the fundamental property of organic substance, 



variability, a process of transformation has been 



possible that maintained organisms even amid the 



changing conditions of the surface of the earth. We 



recognised natural selection as the power that 



continuously adapted living things to their new 



environment. And as we came to the conclusion 



that organisms arose entirely, or at least mainly, from 



adaptations, we are bound to say that selection alone 



has done all the work of transforming life. 



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