142 THE ORIGIN AND EVOLUTION OF LIFE 



The Visible Body The Invisible Germ 



The evolution of somatic (z*. e., The evolution of heredity- 

 bodily) FORM and FUNCTION as ob- chromatin as inferred from the in- 

 served in anatomy, embryology, pa- cessant visible evolution of Form 

 [geontology, and physiology. The and Function. The rise and decline 

 rise, differentiation, and change of of potentialities, predispositions, and 

 function in bodily characters. other germinal characters. 



A clear distinction exists between the slow, stable heredity- 

 chromatin, or germ evolution, and the unstable body cell evolu- 

 tion as viewed by the experimental zoologist. The body is un- 

 stable because it is immediately sensitive to all variations of 

 environment, growth, and habit, while the chromatin alters very 

 slowly. The peculiar significance of heredity-chromatin, when 

 viewed in the long perspective of geologic time, is its stability 

 in combination with incessant plasticity and adaptability to 

 varying environmental conditions and new forms of bodily 

 action. Chromatin is far more stable than the surface of the 

 earth. Throughout, the potentiality of constant changes of 

 proportion, gain and loss of characters, genesis of new charac- 

 ters, there is always preserved a large part of the history of 

 antecedent form and function. In the vertebrates chromatin 

 evolution is mirrored in the many continuous series of forms 

 which have been discovered, also in the perfection of mechani- 

 cal detail in organisms of titanic size and inconceivable com- 

 plexity, like the dinosaurs among reptiles and the whales among 

 mammals, which rank with the Sequoia among plants. 



Adaptive Characters of Internal-External Action, 

 Reaction, Interaction 



Of the causes^ of this slow but wonderful process of chroma- 

 tin evolution there are two historic explanations, each adum- 

 brated in the Greek period of inquiry. 



' See Preface, p. ix. 



