10 INTRODUCTION. 



fixed in their location, like the oyster, move freely some 

 parts of their bodies in obtaining their food. 



Every one knows that animals feel, that they respond 

 to stimuli from without and to impulses from within, and 

 that the most familiar animals exhibit varying degrees of 

 instinct and intelligence. 



These plain facts are fundamental. All the phenomena 

 of animal life may be grouped under four heads : 



1. All those processes which are concerned with feeding 

 and growth, the taking of food, the preparation of it for 

 the use of the body, the carrying of it in solution to the 

 various parts, the building of it into the body structure, 

 the exposure of it to oxygen derived from the air, and the 

 removal of useless portions and of Avaste materials from 

 the body (whether performed by simple means or by com- 

 plex, whether without organs or by means of many separate 

 organs), all may be called phenomena of Nutrition. 



2. All the phenomena connected with the reproductive 

 process may be called phenomena of Reproduction. 



3. All the phenomena of free and spontaneous move- 

 ment may be called phenomena of Voluntary Motion. 



4. All the phenomena of the senses of instinct and of 

 intelligence may be called phenomena of Sensation. 



It will then need to be borne in mind that this fourfold 

 division is made for convenience ; that the four groups of 

 phenomena are not entirely distinct, but interrelated and 

 interdependent. 



In the beginning we must distinguish between that which 

 is necessary and that which is only accessory in animal 

 structure. The robin has a beak for eating, and wings for 

 getting about ; but beak and wings are not necessary for 

 animal existence. Eating and moving are necessary, but 

 these might be effected in other ways. The robin is a bird 

 because of its beak and wings, an animal only because it 

 eats and moves at will. 



