PREFACE v 



as possible the doctrine of evolution. Finally, the last chapter 

 deals with the historical development of the science of zoology. 

 It should aid in giving the pupil the necessary perspective for 

 appreciating the processes by which this organized body of 

 knowledge came into existence. 



The principles of physiology are reserved for full explanation 

 in the study of the earthworm in Chapter XVI. We desire here 

 to express our deep obligation for the inspiration and example 

 of a similar study presented in General Biology by Professors 

 W. T. Sedgwick and E. B. Wilson. We are indebted to them 

 and their publishers for several figures illustrating the develop- 

 ment of the earthworm. Our purpose in delaying the scientific 

 treatment of the principles of physiology, instead of discuss- 

 ing them in the first chapters, is a double one. In the first 

 place, too much abstraction at the beginning of an exposition 

 is likely to prove discouraging to even a willing student ; in the 

 second place, the conception of physiological processes, it seems 

 reasonable to believe, can be formed best after the learner has 

 some knowledge of systems of organs, including, of course, 

 an elementary knowledge of their uses. It then matters little 

 whether the student learns the more detailed facts of physiology 

 from the earthworm or from some other animal, and it even 

 matters little whether or not he actually sees all the organs in 

 the animal where the processes are described as taking place. 



We believe that the attention given in this book to the subject 

 of animal ecology will prove to be justified. Although the eco- 

 logical sections are set off from the other topics, it should not 

 be forgotten that ecology, or the relation of organisms to their 

 environment, has significance only when the subject is considered 

 from the standpoint of the adjustment or the adaptation in the 

 form of the parts of organisms to the life they carry on. 



We have been to considerable pains to produce original figures 

 that should be comprehensive as well as accurate, and, where the 

 subject permitted, artistic. In this we have been assisted by 

 three gentlemen whose ability is e'vident in their work: Mr. S. F. 



