THE LAWS OF ADAPTATION 157 



horizontal steering of the body by means of the median sys- 

 tem of fins. 



The terrestrial and aerial evolution of the four-limbed 

 types (Tetrapoda) is designed chiefly to overcome the resis- 

 tance of gravity and in a less degree the resistance of the atmos- 

 phere through which the body moves. When the aerial stage 

 evolves, with increasing speed the resistance of the air becomes 

 only slightly less than that of the water in the fish stage, and 

 the warped surfaces, the entrant and re-entrant angles evolved 

 by the flying body are similar to those previously evolved in 

 the rapidly moving fishes. 



In contrast with this convergence brought about by the sim- 

 ilarity above described of the physicochemical laws of action, 

 reaction, and interaction, and the similarity of the mechanical 

 obstacles encountered by the different races of animals in 

 similar habitats and environmental media, is the law of diver- 

 gence. 



Branching or Divergence of Form, the Law of Adaptive 



Radiation 



In general the law of divergence of form, perceived by La- 

 marck and rediscovered by Darwin, has been expanded by 

 Osborn into the modern law of adaptive radiation, which ex- 

 presses the differentiation of animal form radiating in every 

 direction in response to the necessities of the quest for nour- 

 ishment and the development of new forms of motion in the 

 different habitat zones. The psychic rudiments of this ten- 

 dency to divergence are observed among the single-celled Pro- 

 tozoa (p. 114). Divergence is constantly giving rise to differ- 

 ences in structure, while convergence is constantly giving rise 

 to resemblances of structure. 



The law of adaptive radiation is a law expressing the modes 



