VII. THE MECHANISM OF ADAPTATION 393 



Organisms which can establish rejective adaptation to injurious 

 agents are, of course, being fitted for existence and, will continue to 

 exist, and therefore this adaptability has to be highly advanced. 



In higher animals including man various hormone systems are 

 amazingly developed for their evolution, and adaptive evolution appears 

 to be mainly achieved by hormones as will be described later. Hor- 

 mones, as mentioned above, can be regarded as protoplasm structures 

 changed by environmental factors. Namely, changed structures of the 

 protoplasm liberated as free forms may be looked upon as hormones, 

 but as the influence of the environment may be greatly modified 

 through hormones, and as the types of hormones are restricted in 

 number in higher animals, the direction of evolution tends likewise to 

 be restricted, not very specifically controlled by environment. 



