THE ORDER AMONG ANIMAL TYPES 95 



as a whole is essential to an appreciation not only of detailed bio- 

 logical principles, but also to a recognition of the unity of animal 

 life. The next task at hand is, then, to acquire a working knowledge 

 of the scale of animal complexity, so that in future pages one may 

 by constant references to the position of animals and organ systems 

 in this scale analyze their significance. 



The branch of Biology which concerns itself with the laws and 

 principles of classifying animals, that is, of arranging them into 

 groups which express the relations of the widely diverse forms to 

 each other, is termed taxonomy. It involves careful analyses of the 

 structure and development of organisms in order that they may 

 be compared. On the bases of such comparisons animals are ar- 

 ranged into groups based on the relative extent of their similarities 

 and dissimilarities. Observable likenesses and unlikenesses serve as 

 the diagnostic characters in the arrangement of the groups. The 

 system of classification that is used today is the result of long years 

 of study and effort and has come to serve as a universal basis 

 wherever animals are studied. Objections are often raised that 

 biologists employ an unnecessary number of complex and unusual 

 terms as the names of various forms and structures and of various 

 processes which go on in the organism. A detailed defense of the 

 system is out of place here, but it may be pointed out that a system 

 of nomenclature is necessary. This is apparent to anyone after even 

 a brief experience in the field, for animals, structures, and processes 

 outside the experience of everyday life are encountered. Out of the 

 system has grown a language of the science. It is as necessary to a 

 knowledge of Biology to learn its language as it is necessary to 

 learn the vocabulary of a foreign tongue in order to appreciate its 

 literature. 



Basis of Classification. Starting with the fact already known 

 to the reader that living objects may be arranged according to their 

 very broadest similarities into two great groups or kingdoms, Plant 

 and Animal, the Animal kingdom has been further divided into two 



