INTRODUCTION XI 



The theory leaves unexplained how bodily modifications effect correspond- 

 ing changes in the germ cells which transmit inherited traits. On the 

 whole, experimental evidence does not support the assumption of the 

 inheritance of bodily modifications. 



2. According the Darwin's hypothesis of natural selection, four 

 factors effect organic evolution : 



1. Variation 



2. Multiplication 



3. Competition 



4. The inheritance of useful variations 



The theory asserts that no two individuals are precisely alike, and that 

 many more are born than can possibly live. The result is a struggle for 

 existence so severe that only those types survive whose variations favor 

 them in the struggle. These transmit their favorable traits through 

 heredity to their offspring. Carried on through many thousands of 

 generations, such changes would, it is assumed, eventually produce new 

 species. 



3. During the twentieth century, Hugo De Vries has advanced a third 

 view, which he calls the Mutation Theory. According to the Mutation 

 Theory, new species arise suddenly, discontinuously, not, as Darwin 

 thought, by slow accumulation of slight differences. De Vries conceives 

 of an organism as a mosaic of traits. A new combination of characteristics 

 constitutes a new species. Breeding evening primroses in his Amsterdam 

 garden, De Vries discovered that mutations are inherited. In other 

 words, mutants breed true. After it arises, a mutant species is subject 

 to natural selection or elimination, but its origin, like that of the fluctuat- 

 ing variations of Darwin, is not dependent upon this struggle for existence. 

 The factors in evolution, therefore, according to De Vries are: 



1. Mutation 



2. Heredity 



The laws of heredity are found to be in accord with the mutation theory. 

 The theory has, however, been criticised as failing to explain the origin 

 of adaptive mutations. Since the cause of adaptive change must affect 

 the germ-cell in order to be inherited, and since biologists are still searching 

 for the factors which determine adaptive mutation, it must be admitted 

 that the cause or causes of evolution are unknown. Of the three hypo- 

 theses mentioned, the Lamarckian is least acceptable to biologists. 



At the present time two divergent conceptions of evolution are held by 

 biologists — mechanical and emergent. 



According to the mechanical conception of evolution, the universe is 

 a machine operating in accordance with "immutable" laws. The entire 

 universe or any part of it consists of particles in motion grouped into 



