THEORIES OF WEISMANN AND DE VRIES 495 
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. 
I]. 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 sclects 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, 
4.0; 
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. 
