No. 643] EFFECTS OF ENVIRONMENT 153 



dying out of extinct types of animals is ''over-adapta- 

 tion," or better ''too mnch specialization." A system of 

 activities, as represented by an organism, can not depend 

 absolutely on another system of activities, as represented 

 by environment. The organism changes and the environ- 

 ment changes too. If the environment continues for a 

 time in a fairly stable condition, an animal may become 

 adapted to it to such a degree that, if the environment 

 then does change, the animal can not respond enough to 

 continue to live. The wood frogs in the United States 

 breed when the water is at freezing temperatures ; frogs, 

 belonging to the same genus as the wood frog, that live in 

 Cuba die when the temperature falls below seven degrees. 

 These frogs are adapted to different environments and 

 those in Cuba will be in greater danger of extinction if 

 there is a prolonged cold period. 



There is a general tendency among animals to find suc- 

 cess during conditions of stability. Certain arthropods 

 left the water and attained stable respiratory conditions 

 and freedom from water-soluble poisons by going on 

 land. Later, certain of these arthropods again gained a 

 thermally stable environment in the water and continued 

 to enjoy a stable gaseous environment by carrying air 

 into the water with them. When any race of animals 

 attains a stable environment, it may become specialized 

 to it. We see a manifestation of the same type in the 

 psychology of man. It is "human nature" to desire 

 stability— to be free from care and worry; to know where 

 one stands. 



On the other hand, continual change is a stimulus to 

 progressive response— in fact, one is tempted to say that 

 lack of change is injurious to living organisms and that 

 changes often stimulate living systems to renewed activ- 

 ity. Pa>Tie kept fruit flies continuously in the dark; 

 Calkins and Woodruff maintained protozoans on unvary- 

 ing culture media. All these investigators agreed that 

 lack of variation in the environment was injurious. This 

 raises a dilemma— on one hand animals tend to become 

 highlv adapted (or specialized) when the environment is 



