INTRODUCTION 3 



all the other fields of zoological investigation. From a careful 

 examination of fossil forms, it is believed that the horse originated 

 from a five-toed ancestor. This theory can be supported by 

 evidence gained from the study of the embryology of modern 

 horses. The physiology and ecology of horses give further 

 evidence as to why some of the horse-like animals persisted to 

 the present time while others became extinct, and why horses 

 are now found in some regions of the earth and not in others. 



2. CLASSIFICATION 



Animals are not infinitely variable. Their investigation has 

 resulted in the establishment of about five hundred thousand 

 species. These are grouped, according to similarity in structure, 

 into about fifteen large divisions, or Phyla. One of the chief 

 aims of zoologists in the past has been to give a complete descrip- 

 tive inventory of the animal kingdom. The large number of 

 species which have been described has made it necessary to sepa- 

 rate each of the phyla into smaller and smaller subdivisions. 

 Those animals which are most nearly related have therefore been 

 placed together, and certain definite names are now generally 

 adopted for use in classification. Thus under each phylum one 

 or more classes are included, and under each class, a variable 

 number of orders. In the following list of such terms the 

 groups become successively smaller: phylum, class, order, family, 

 genus, species, individual. 



An example will perhaps make the system more clear. George 

 Washington was an individual; he belonged, with other men, 

 to the species sapiens of the genus Homo. This genus, together 

 with another of somewhat questionable relationships, the extinct 

 Pithecanthropus, constitutes the family Hominida. The Homi- 

 nid(E are included with ten other families of monkey-like animals 

 in the order Primates. Fifteen related orders, of which the Pri- 

 mates form one, are placed in the class Mammalia. All mammals 

 possess hair and mammary glands; these characteristics dis- 



