THE E V L U T I O N OF LIFE 



by Darwin's theory of selection. Its real foundations 

 are these three phenomena: heredity, adaptation, and 

 the struggle for existence. All three are, as I have 

 often said, of a purely mechanical and not a teleological 

 nature. Heredity is closely bound up with the physio- 

 logical function of rcj)roduction, and adaptation with 

 nutrition; the struggle for life follows logically and 

 mathematically from the disproportion between the 

 number of potential individuals (germs) and of actual 

 individuals that grow to maturity and proi)agate the 

 species. 



When I had, in my General Morpholof^y, endeavored 

 to gain acceptance for Darwin's theory of selection, and 

 had presented evolution as a comprehensive theory from 

 the point of view of the monistic philosophy, a number 

 of works, sometimes of value, appeared, which made 

 special studies of the various parts of the immense prov- 

 ince. Eighteen years afterwards a greater work was 

 published, which started from the same monistic prin- 

 ciples, but reached the same conclusion by a difTerent 

 way. In 1884 Carl Nageli, one of our ablest and most 

 philosophic botanists, issued his M echanical- physiological 

 Theory of Evolution. This interesting book consists of 

 various parts. It is especially notable that evolution is 

 presented in it as the one possible and natural theorv of 

 the origin of species ; even morphology and classification 

 are treated explicitly as " phylogenetic sciences." The 

 chapter on archigony — a dark and dangerous problem 

 that is generally avoided by scientists! — is one of the 

 best that has been written on the sul)ject. On the other 

 hand, Nageli rejects Darwin's theory of selection alto- 

 gether, and would ex])lain the origin of species by an 

 inner "definitely directed variation," independently of 

 the conditions of existence in the outer world. As Weis- 

 mann has properly observed, this internal principle of 

 evolution, which dispenses with adaptation in the true 



3^5 



