CHAPTER VIII. 



HOW ANIMALS KNOW THINGS AND CONTROL 

 THEIR MOTIONS. 



Thus far we have considered the mechanisms animals 

 have for motion and for obtaining oxygen and food. A 

 more difficult but more interesting subject is how motions 

 take place in the animal, how they are guided, how they 

 are stopped, in short, how the whole conduct of the life of 

 the animal is carried on. To understand better what 

 these processes consist of, let us consider as an example 

 the life of a common bird. We know that after hatching 

 from the egg it takes food, learns the notes of the parent 

 bird, learns to fly, learns to fight or to avoid enemies, all 

 these including motions guided by sight, hearing, touch, 

 and smell. On the approach of winter it migrates to the 

 south; in spring it returns, chooses a mate, builds a nest, 

 and rears young to which it teaches in turn the ways of 

 bird life. While the full explanation of these processes 

 is far from being reached, and while we cannot here dis- 

 cuss them at length, yet we may at least examine some 

 of the parts ot the body specially concerned with these 

 processes. In the higher animals they are determined 

 and directed by means of the sense-organs and the ner- 

 vous system. In vertebrates the special senses, as they 

 are called, are those of sight, hearing, smell, taste, 

 touch, cold, heat, and one called the muscular sense. 

 A part of the eye known as the retina is specially 



99 



