PRINCIPLES OF 



ANIMAL BIOLOGY 



CHAPTER I 

 INTRODUCTION 



Zoology is the science of animals. This definition is brief and per- 

 haps vague, but it is doubtful whether it can be improved upon without 

 lengthy explanations. This was the definition centuries ago, when 

 zoology was almost exclusively the classification and naming of animals. 

 Since that time there has arisen a vast body of doctrine concerning 

 modes of life, life processes, inter-relations of animals, development, 

 distribution, and descent, most of which has little direct bearing upon 

 classification. In many cases the principles involved are identical, or 

 very similar, in both plants and animals, and the term biology is used 

 to designate them. For this reason, some have preferred to call the 

 science that embraces the body of fundamental doctrine just mentioned, 

 as applied to animals. Animal Biology. The latter name appears to con- 

 vey less of the idea of classification, and more of principles, than does 

 the term zoology. In the last half century, however, classification of 

 animals has come to be founded upon principles as basic as those under- 

 lying the other branches of the science. It is a matter of slight impor- 

 tance, therefore, which name is adopted. 



The definition of zoology given above, as will be readily understood 

 from the brief list of subjects which it has come to include, must be in- 

 terpreted broadly. Anything that has to do with animals is part of 

 zoology. A definition that conveyed to the uninitiated an adequate con- 

 ception of the content of the science would be indeed voluminous. This 

 book is one long definition of zoology. Without waiting, however, for 

 the last chapter, it will be useful to attain a perspective at the very out- 

 set by stating in general but concrete terms what the science of zoology 

 comprises. 



Morphology. — The structure of animals has been until recently the 

 chief interest of a large majority of zoologists. The branch of zoology 

 deahng with structure is called morphology. Gross form of the body 

 as a whole, the structure of its parts or organs or systems of organs, the 

 structure of the tissues that make up the organs or bind them together, 

 the structure of the units or cells which compose the tissues, apd the 

 structure of the minute components of the cells, are all included in mor- 



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