PREFACE 



attempted to combine factual information with the interpretations and con- 

 clusions which the facts justify and which give point and meaning to the 

 facts as presented. 



The approach of the present work, like that of the Curtis and Guthrie 

 series, is to begin with the vertebrate type of organization, introducing the 

 student to various aspects of structure and function in vertebrates and then 

 proceeding to a stepwise consideration of how the vertebrate organization 

 developed through evolution. The early chapters, although generally ori- 

 ented toward the vertebrates, deal broadly and comparatively with such sub- 

 jects as cellular structure and function and the universal aspects of metabolism 

 and responsiveness. Accordingly, these chapters furnish an introduction to 

 principles that apply throughout the Animal Kingdom. 



The organization of this book has been made flexible enough to permit 

 different manners of presentation. A teacher may elect to follow the order 

 of the chapters, or to begin with classification (Chapter 7), study the inverte- 

 brate groups, and then return to the vertebrates. Depending on the organiza- 

 tion of his course, the teacher must decide when the general chapters — those 

 dealing with classification, heredity, ecology, and evolution — can be most 

 appropriately integrated. The broadly comparative survey of invertebrate 

 organ systems and vital functions (Chapter 17) may be particularly useful in 

 courses taking formal notice of only a few of the invertebrate groups and will 

 serve as a basis for broad comparisons if only vertebrates are studied in detail. 



To those teachers who favor a man-centered approach to zoology, the ex- 

 tended treatment given the invertebrate groups in this book may seem un- 

 warranted. To us it appears that since the vast majority of animals are 

 invertebrates, to ignore them is to lose sight of man's relation to the animal 

 world and of the teeming mass of more primitive types from which the verte- 

 brates have so recently emerged. The chapters on invertebrate phyla do not 

 merely form a systematic survey of the Animal Kingdom but present a careful 

 analysis of each group, its significant structural and functional aspects, and 

 its place in phylogeny. The unifying theme of this section of the book is the 

 progression of levels of organization through evolution. 



The several chapters discussing the representative types of animals regularly 

 include remarks on their activities and habitat relations. In addition, a 

 separate chapter has been devoted to ecology. Here we discuss the short- 

 range and long-range interactions between animals and their environments, 

 the specific eff^ects of many environmental factors in determining the survival 

 and distribution of animals, and the concepts of competition and the balance 

 of nature. This material is, in part, a preparation for the following chapter 

 in which the role of the environment in evolutionary change is discussed. 

 We hope that this chapter on ecology fills the gap reported by many who have 

 used the Curtis and Guthrie textbooks. 



A major problem on which there is little agreement among teachers of 

 zoology has been aptlv termed "the chemistry dilemma." Students of be- 

 ginning zoology commonly have little if any background in chemistry, yet 



