CHAPTER I. 

 INTRODUCTORY. 



WE live in a world that is teeming with life. The air, the 

 surface of the land, the waters of ocean, river, and pond, swarm 

 with living organisms, each more or less perfectly adapted to the 

 conditions of its existence. Many problems arise with regard to 

 this world of living things, What is their form and structure ? 

 How do they move and breathe and reproduce their kind ? How, 

 and on what do they feed ; and how does the food minister to 

 their growth or their activity ] How are they distributed over 

 the earth's surface ? What is the method and manner of their 

 origin ? These, and other questions of like nature, arise in con- 

 nection with the world of life. And the science that deals with 

 these problems is the Science of Biology. 



This science branches into two main departments, in accord- 

 ance with the division of living things into Animals and Plants. 

 The two branches start, indeed, from a common stem, for there 

 are certain characteristics common to plant life and animal life, 

 and the lowest plants are scarcely to be distinguished from the 

 lowest animals. But the structure and functions of the vast 

 majority of animals differ so markedly from the structure and 

 functions of the vast majority of plants, and the problems of 

 animal life differ so materially from the problems of plant life, 

 that the Science of Animal Biology, which deals with the former, 

 is justly entitled to a distinct position as a separate branch of 

 study. It is with this branch that this volume deals. 



The essence of science is organisation and exactness. Most 



