390 THEORIES OF DEVELOPMENT 



come plants, the individuals of the successive generations." 

 He further states "that only those characters are transmissible 

 which have been controlled — i. e., produced — by determinants of 

 the germ, and that consequently only those variations are 

 hereditary which result from the modification of several or many 

 determinants in the germ plasm, and not those which have 

 arisen subsequently in consequence of some influence exerted 

 upon the cells of the body. In other words, it follows from this 

 theory, that somatogenic or acquired characters cannot be trans- 

 mitted. 



"This, however, does not imply that external influences are 

 incapable of producing hereditary variations; on the contrary, 

 they always give rise to such variations when they are capable 

 of modifying the determinants of the germ plasm. Climatic 

 influences, for example, may well produce permanent variations 

 by slowly causing gradually increasing variations to occur in 

 the determinants in the course of generations. The primary 

 cause of variation is always the effect of external influences. 

 When deviations only affect the soma they give rise to temporary 

 non-hereditary variations; but when they occur in the germ 

 plasm they are transmitted to the next generation and cause 

 corresponding hereditary variations in the body." 



The Mutation Theory. — De Vries is the chief exponent of the 

 mutation theory, though Bateson also emphasizes its import- 

 ance. This theory assumes that new species and varieties are 

 produced from existing forms by sudden leaps. These may 

 arise simultaneously and in groups, or separately and at more or 

 less widely distributed periods. This new theory does not 

 try to account for these sudden variations, but claims that 

 "when they occur it is a striking fact that the characters tend 

 to be transmitted." When a mutation appears, it will survive 

 and leave descendants if it is adapted to its environment. Only 

 a comparatively small amount of evidence has been found to 

 support the zoologic side of proof for this theory. 



Orthogenesis is believed by many specialists, among whom 

 may be mentioned Eimer, Whitman, Tower, and Ruthvcn. 

 Development in a definite, predetermined direction, even if 

 this development is harmful to tlie race, is called orthogenesis. 

 "According to this theory, certain lines of development remain 



