RELATIONS AND SURROUNDINGS. 127 



are numerous special adjustments to the particular types 

 of external conditions. For example, the eye of an 

 organism that lives in water will differ somewhat from eyes 

 used in the atmosphere; the stomach of a vegetable feeder 

 will be constructed differently from that of a flesh eater; 

 animals which use the air for locomotion must develop 

 certain adjustments not found in those that walk upon 

 solids. 



The following outline of the principal classes of adapta- 

 tions found in animals will aid the student in realizing 

 how universal this principle is and how powerful the 

 environment is in controlling the nature and form of 

 organisms. 



A. Adaptations to the fundamental conditions and 

 activities of life. (The illustrations are not in- 

 tended to be exhaustive; the student should 

 find others.) 



1 . To Food. The sense of taste and smell ; hunger ; 

 the organs for capture of food, for digestion, 

 absorption, etc. 



2. To Oxygen. Lungs, gills, skin, etc. 



3 . To Moisture. Sense of thirst ; power of absorp- 

 tion; coverings to prevent the evaporation of 

 contained water; encystment and resting stages 

 to resist drouth. 



4. To Temperature. Oxidation and animal heat; 

 hair, fur, feathers; perspiration and its evapora- 

 tion; hibernation, resting stages, winter eggs; 

 migration. 



5. To Gravity. -The sense of equilibrium; appen- 

 dages; the supporting parts of the body. 



