XIV INTRODUCTION. 



and although the most common of all others, they are not 

 the easiest to be clearly understood. The Genus is founded 

 upon some of the minor peculiarities of anatomical struc- 

 ture, such as the number, disposition, or proportions of the 

 teeth, claws, fins, &c., and usually includes several kinds. 

 Thus, the lion, tiger, leopard, cat, &c., agree in the struc- 

 ture of their feet, claws, and teeth, and they belong to the 

 genus Felis ; while the dog, fox, jackal, wolf, &c., have 

 another and a different peculiarity of the feet, claws, and 

 teeth, and are arranged in the genus Canis. 



The Species is founded upon less important distinctions, 

 such as color, size, proportions, sculpture, &c. Thus we 

 have different kinds, or species, of duck, different species of 

 squirrel, different species of monkey, &c., varying from 

 each other in some trivial circumstance, while those of 

 each group agree in all their general structure. The spe- 

 cific name is the lowest term to which we descend, if 

 we except certain peculiarities, generally induced by some 

 modification of native habits, such as are seen in domestic 

 animals. These are called varieties, and seldom endure 

 beyond the causes which occasion them. 



Several genera which have certain traits in common are 

 combined to form a family. Thus, the alewives, herrings, 

 shad, &c., form a family called Clupeidse ; the crows, 

 blackbirds, jays, &c., form the family Corvidae. Families 

 are combined to form orders, and orders form classes, and 

 finally, classes are combined to form the four primary divi- 

 sions of the Animal Kingdom, namely, the departments. 



For each of these groups, whether larger or smaller, we 

 involuntarily picture in our minds an image, made up of the 

 traits which characterize the group. This ideal image is 

 called a TYPE, a term which there will be frequent occasion 

 to employ, in our general remarks on the Animal Kingdom. 

 This image may correspond to some one member of the 



