THE DEGREES IN CLASSIFICATION. 127 



usually very difficult to decide which belong to the same 

 family. A family is a division by relationship. The members 

 need not be of the same size or color, but they usually have 

 the same look or similar habits. Who would not say that 

 all the horned owls belonged to one family? They may be 

 very unlike in ^ize, but they have the family nose and eyes. 



The next division below a family is a genus. A genus com- 

 prises those kinds so closely related that we judge they may have 

 had a common ancestor. Birds of the same genus are usually 

 nearly alike in size and often have some peculiar color pattern 

 which makes them look alike, but the surest marks are re- 

 semblances in feet, bill, wings, and tail. 



A species is the next and final division. We cannot go any 

 farther in our classification. A species includes all the birds 

 so near alike that they cannot be told apart by any permanent 

 pecnliarity. Each species has its own marks, colors, and shapes, 

 and mates with its own kind, never (except by a rare accident) 

 with any other species. 



A\'hen we have separated our specimens into species, we 

 have found the unit of classification. 



