326 DARWINISM TO-DAY. 



"The only general view, as it seems to me, which can be held 



concerning the structural diversity of the animal kingdom, is to 



regard it as resultant of two more or less opposing 



Cunningham _ general tendencies. On the one hand, there is uni- 

 and orthogenesis, . . , . 



versa! evidence of a tendency to definite variation, or 



growth in different directions, leading to manifold variety of regu- 

 lar definite symmetrical forms. This tendency can only be regarded 

 as internal to the organism, as connected with a tendency to growth 

 and multiplication inherent in organic units. On the other hand, 

 there is the molding, limiting, constructing action of the external 

 forces of the environment resulting in more or less complete adap- 

 tation. Whatever be the process of adaptation, whether Darwinian 

 selection or Lamarckian modification, adaptive structural combina- 

 tions are mechanisms each working with the particular result which 

 is important to the feeding, living, and breeding of the organism. 

 Whatever the causes of non-adaptive variation, the resulting struct- 

 ural features are the regular genetic forms and characters which the 

 multitude of different organic forms present in such marvellous 

 diversity. No one who, like Weismann, ignores everything except 

 adaptation, or who, like Bateson, regards the study of adaptations 

 as barren and profitless, can hope to produce a consistent and com- 

 prehensive theory of organic evolution." 



24 Delage, Yves, "L'Heredite," 2d ed. p. 849, and others, 1903. 



25 Jaeckel, O., "Uber verschiedene Wege phylogenetischer Ent- 

 wicklung," 1902. 



