Organs and Classification of Animals. II 



purposes of study. Those individuals which are 

 so far identical in- structure as to lead us to believe 

 that they are descended from common parents we 

 speak of as belonging to the one species. Species 

 is thus our unit in systematic zoology, but as two in- 

 dividuals are seldom absolutely identical in all respects 

 specific distinctions must be more or less arbitrary. A 

 group of allied species embodying the same structural 

 ideas is called a genus. An assemblage of allied 

 genera is a family ; a group of related families make 

 up an order ; while related orders make up a class, and 

 the several classes included in the animal kingdom 

 are united in certain primary categories called sub- 

 kingdoms. Systematic zoologists give a Latin name to 

 each of these, and for convenience each species is 

 designated by a Latin word to which is prefixed the 

 name of the genus. The specific name is generally 

 an adjective, the generic is a substantive, and should 

 be written with a capital letter. Thus the dog is called 

 by zoologists Canis familiaris, Canis being the generic, 

 familiaris the specific name. Canis aurats is the 

 jackal, Canis lupus the wolf. That species in a genus 

 which most strikingly embodies the generic characters 

 is the type of the genus. We also speak of the type of 

 a family, of an order, or of a class, the type being that 

 species which displays most clearly the characters of 

 the group ; and for convenience we attribute certain 

 characters to ideal types to illustrate truths in classifi- 

 cation. 



The type genus usually gives its name to the family , 

 thus the dog-family is called Canidae. 



Homology. In comparing animals, the most im- 



