20 ESSENTIALS OF ZOOLOGY 



C. Cytology is the study of the minute structure of the cells which, 

 we will learn, are the units of structure of all living matter. This 

 field of study has yielded many fundamental concepts of the factors 

 involved in the living process. 



2. Taxonomy is the subdivision which deals with the classifica- 

 tion or orderly arrangement of organisms according to their natural 

 relationships. This field is often spoken of as systematic zoology. 

 The number of described species of animals as given by different 

 authorities ranges from 840,000 to well over a million. One well- 

 known writer says there are probably no less than 2,000,000 species 

 of living animals Besides these, there are large numbers of extinct 

 forms. It can readily be seen that a system for putting these large 

 numbers of different kinds of animals into a known order is one of 

 the first prerequisites for dealing with them. On a much smaller 

 scale, the department store is systematized for some of the same 

 reasons. One can see that it would be next to impossible to do 

 business if a company were to provide a large floor space, go out 

 and buy the thousands of different kinds of articles that are handled 

 by a department store, and just throw all of them on its floors at 

 random. 



The relationships of animals are discovered from similarity of 

 structure, from facts of distribution, from embryological similari- 

 ties, and many other comparisons. A group in which the members 

 are very closely related is likely to be comparatively small. These 

 groups are ranked together according to evident relationships. 

 Zoologists recognize a number of large divisions of the animal king- 

 dom based on certain general characteristics. Each of these divi- 

 sions is known as a phylum and is divided into classes, each class 

 is divided into orders, each order into families, each family into 

 genera, and each genus into species. Taking the classification of 

 man as an example we have : 



Phylum: Chordata 



Subphylum : Vertebrata 

 Class: Mammalia 



Order : Primates 



Family: Hominidae 

 Genus: Homo 



Species: sapiens 



