300 COOK 



Instead of slow or gradual changes of the characters of species 

 there are supposed to occur at remote intervals in the life of a 

 species relatively brief periods of mutation in which violently 

 abrupt variations are given off in an explosive manner. Each 

 of these discontinuous variations is considered as representing 

 the production of a new species, there being no gradations be- 

 tween it and the parental type. Unfortunately, the wide appli- 

 cation of this analogy is prevented by the fact that in many 

 natural groups descent from a single individual is impossible. 

 Moreover, the new types or sports studied by Professor De 

 Vries are, like other closely inbred plants and animals, much 

 less fertile than their wild progenitors, thus increasing the 

 probability that the inbreeding or segregation necessary to 

 secure and preserve these abnormalities would give them a 

 fatal handicap in the struggle for existence. Finally, the wide 

 distribution, among both plants and animals, of sexual differen- 

 tiation and other expedients for securing cross-fertilization, seems 

 a sufficient warrant for distrusting any theory which disregards 

 this important group of evolutionary phenomena. 



Determinant Theories. — The noninheritance of acquired 

 characters led Nageli and Weismann to formulate what maybe 

 termed determinant theories, under which the motion of species 

 is not thought of as caused or directly influenced by the environ- 

 ment, but as the function of internal " mechanisms of descent." 

 Nageli believed that species did not vary in all directions indis- 

 criminately, as Darwin had held, but that they kept, without 

 selective influences, a definite direction. He therefore con- 

 cluded that the organization of living matter contained what he 

 called a " Vervollkommungsfirinzifi" or principle of perfection, 

 which carried them ever upward along the road from simplicity 

 to complexity. 



Weismann sought in his doctrine of determinants to render 

 this conception more concrete regarding the nature of the in- 

 ternal mechanism, and to provide a means of selective influence. 

 Determinants may be described as biological atoms, resident in 

 reproductive cells and able to determine in advance the charac- 

 ter of the new organism, independent of its environmental rela- 

 tions. The environment also has no effect on the next genera- 



