306 cook 



of unknown character. Selection can determine the survival 

 of mutations adapted to environmental conditions, but exerts no 

 direct adaptive influence. 



Determinant theories view species as moving gradually in 

 definite directions in obedience to internal " principles of per- 

 fection " or "mechanisms of descent." Adaptation depends 

 on the coincidence between evolution and environment ; selec- 

 tion exerts no direct influence. 



Kinetic theories view species as normally in motion, but not 

 in a single or definite direction, and not as a result of environ- 

 mental causes. The normal evolutionary motion of the species 

 may be restricted and deflected by the selective action of the 

 environment, resulting in adaptation. 



The adjacent tables may assist in showing the relations be- 

 tween these different types of evolutionary theories. Table I 

 indicates the methods by which the various doctrines answer 

 some of the principal questions regarding evolutionary motion. 

 Table II brings these questions into relation with the conclu- 

 sions reached in previous chapters. Discrepancies between dif- 

 ferent evolutionary doctrines are often explainable by the fact 

 that some of them are in reality theories of adaptation or of 

 speciation, rather than of evolution. Thus, as the table shows, 

 interbreeding is a strongly negative factor in the multiplication 

 of species (speciation), but at the same time it is a strongly posi- 

 tive factor in evolution. The chief factors in adaptation and 

 speciation have only negative or restrictive effects upon evolution. 



NORMAL CONDITION OF SPECIES. 



The most fundamental diversity of opinion regarding the 

 nature of evolutionary motion is that of the normal condition of 

 species. Two assumptions are possible and have equal warrant 

 for scientific consideration. Under theories of environmental 

 and selective causation, it has been taken for granted that species 

 are normally stationary and uniform unless acted upon by some 

 disturbing external influence. The question of causes, on this 

 assumption, is a simple one. The difficult problem is to explain 

 how the external influences produce the organic results which 

 have been ascribed to them. Fifty years of study have been 



