PREFACE 



This book is the culmination of more than twenty years of teaching intro- 

 ductory zoology at the college and university level. During this period it has 

 become increasingly evident to me that in order to understand the life sciences, 

 students need, more than anything else, a broad underlying principle to cling 

 to. In biology we have that in the story of organic evolution. Just as in the 

 physical sciences, where the relationship of the elements in the periodic table 

 provides a central core around which all other facts must revolve, so in biology 

 evolution offers a great unifying principle to which all lesser principles can be 

 related. This magnificent idea cannot be relegated to an isolated chapter; it 

 must be the central theme, permeating the entire book, if it is to become a part 

 of the student's thinking as it should. Thus organic evolution is the central 

 theme of this work. 



Every attempt has been made to integrate the seven parts of the book into 

 what amounts to an historical narration of animal life. My aim has been to 

 point out the salient points in zoology without diluting the subject matter; to 

 present important, and often complex, material in a manner that will at the 

 same time fascinate the general education student and challenge the embryo 

 professional zoologist. How well this dual function has been accomplished, 

 students of the future will decide. 



After a brief discussion of the position zoology occupies as a science, there 

 follows in Part I a discussion of the scientist and his methods. It is important 

 to have the student understand the methods of science and recognize that 

 zoology uses the same disciplines employed in other sciences. 



The story of evolution begins to unfold in Part II, beginning with a survey 

 of the early history of the earth and the nature of the physical world. It seems 

 logical to start with the physical setting in which life took its inception. A 

 possible explanation of the origin of the first living thing is offered, and its 

 properties are examined. 



In Part III we speculate upon the transition from single cells to many cells 

 and discuss some of the problems arising out of organization into the more 

 complex forms. The organized animal is considered in relation to its environ- 

 ment. 



