THE CLASSIFICATION OF ANIMALS 51 



of less and less importance, as we descend from the 

 general to the special. 



Individuals are the units of the Animal Creation. 

 An animal existence, complete in all its parts, is an 

 individual, whether separate, as man, or living in a com- 

 munity, as the coral. 7 



Species is the smallest group of individuals which can 

 be defined by distinct characteristics, and which is 

 separated by a gap from all other like groups. A well- 

 marked subdivision of a species is called a variety. 

 Crosses between species are called hybrids, as the 

 mule. 



Genus is a group of species having the same essential 

 structure. Thus, the closely allied species cat, tiger, 

 and lion belong to one genus. 



Family, or Tribe, is a group of genera having a simi- 

 lar form. Thus, the dogs and foxes belong to different 

 genera, but betray a family likeness. 



Order is a group of families, or genera, related to one 

 another by a common structure. Cats,' dogs, hyenas, 

 and bears are linked together by important anatomical 

 features ; their teeth, stomachs, and claws show carniv- 

 orous habits. 



Class is a still larger group, comprising all animals 

 which agree simply in a special modification of the type 

 to which they belong. Thus, fishes, amphibians, rep- 

 tiles, birds, and mammals are so many aspects of the 

 vertebrate type. 



Branch is a primary division of the animal kingdom, 

 which includes all animals formed upon one of the 

 various types of structure ; as vertebrate. 



The branches are grouped into two great Series (Pro- 

 tozoa and Metazoa), according to their histological 

 structure and mode of development. 8 



These terms were invented by Linnaeus, except 



