Summary 429 



studies, in the material of which progressive integration and 

 differentiation are quite as obvious as they are in the evolu- 

 tion of living plants and animals; and perhaps to extend the 

 philosophical method more effectively to the consideration 

 of the field in which physico-chemical methods have hitherto 

 prevailed. 



Summary 



Masses of facts have been accumulated about the living 

 inhabitants of the earth, those of our own times and those 

 of ancient times. About these facts there can be no dispute, 

 they are open to all who have eyes to see. These facts leave 

 no doubt that " evolution " has taken place, that is, that 

 there has been continuity of descent with more or less rapid 

 modification of the forms, among animals and among plants. 

 This conclusion is supported by outstanding facts of struc- 

 ture, of classification, of physiological characteristics and 

 processes, of distribution of forms in time, of distribution of 

 plants and animals on the face of the earth at the present 

 time, of development, of heredity. 



The processes in detail, or the mechanisms that have 

 brought about modification of types in the course of descent, 

 are not altogether clear. Several theories are logical enough 

 in general terms, but are not in agreem^ent with the details of 

 fact. Certainly no one theory of evolution adequately meets 

 all the facts. 



There has been variation, variation is constantly to be 

 observed among all living forms. This variation is indeter- 

 minate, that is, it takes place in all directions, with respect to 

 every character. On the other hand, there is unmistakable 

 evidence of determinate or directed evolution. These two 

 sets of facts, however, are not mutually contradictory, since 

 the two kinds of variation may arise from totally different 

 sources. 



Directed variation, or orthogenetic evolution, has in 

 many cases led to " improvements " or to better adaptation 



