THE HISTORY OF ANIMAL LIFE 393 



peared in the score of years that these animals have been bred in the 

 laboratory. According to De Vries, species have not arisen by grad- 

 ual selection but by jumps through sudden transformations. 



Comparison of the Chief Theories of the Mechanism of 

 Evolution. The main features of these four theories may now 

 be set down, in order that they may be better compared: 



Lm march's Theory: 



( 1 ) Variations occur on the principle of use and disuse. 



(2) Variations are inherited directly and improve with future genera- 

 tions. 



Darwin's Theory: 



(i) Variations occur; no attempt is made to explain their occurrence. 

 (2) Variations are either perpetuated or extinguished by natural se- 

 lection. 

 Wei smart n 's Th eory : 



(i) Germinal material is continuous from one generation to the next 

 and not derived from soma. 



(2) There is no inheritance of acquired characters, no effect of use 

 and disuse; only germinal variations can be inherited and the oc- 

 currence of germinal variations is independent of the environ- 

 ment. 



(3) Variations arise as a result of new combinations in the germ cells. 

 Weismann recognized the possibilities of an almost limitless num- 

 ber of permutations and combinations when the germ cells of 

 two parents become associated in the act of fertilization of the 

 egg and regarded these possibilities as sufficient to account for 

 germinal variations. 



(4) The principle of natural selection by elimination of the unfit 

 operates to determine which germinal characters shall persist. 



De Vries' Theory: 



(i) The formation of new species is due to sudden mutations, that 

 is, the appearance of profound changes and differences between 

 parent and offspring. 

 (2) Natural selection operates to retain or eliminate such mutations. 



Modern Views. The vast accumulation of data on inheritance 

 that have resulted from the experimental testing of the theories of 



