THEORIES OF WEISMAXX AND DE VRIES 45 



I. Lamarck's Theory of Evolution. 



1. Variation is explained on the principle of use and 



disuse. 



2. Heredity: The variations are inherited directly and 



improved in succeeding generations. 

 A long time and favorable conditions are required 

 for the production of new species. 



II. Darwin's Theory of Natural Selection. 



1. Variations assumed. 



2. Heredity: Those slight variations which are of use 



to the organism will be perpetuated by inher- 

 itance. 



3. Natural selection is the distinguishing feature of 



the theory. Through the struggle for existence 

 nature selects those best fitted to survive. The 

 selection of trivial variations that are of advantage 

 to the organism, and their gradual improvement, 

 leads to the production of new species. 



III. Weismann's Theory of Continuity of the Germ-plasm. 



1. The germ-plasm has had unbroken continuity from 



the beginning of life. Owing to its impression- 

 able nature, it has an inherited organization of 

 great complexity. 



2. Heredity is accounted for on the principle that the 



offspring is composed of some of the same stuff 

 as its parents. The body-cells are not inherited, 

 i.e., 



3. There is no inheritance of acquired characters. 



4. Variations arise from the union of the germinal 



elements, giving rise to varied combinations and 

 permutations of the qualities of the germ-plasm. 

 The purpose of amphimixis is to give rise to vari- 

 ations. The direct influence of environment has 

 produced variations in unicellular organisms. 



