352 ANIMAL BIOLOGY 



1. Classification 



When the serious study of biological classification was well 

 under way, biologists found increasing evidence of the similarity, 

 or affinity, of various species of animals and plants. Not only is 

 it possible to arrange animals, for example, in an ascending series 

 of increasingly complex forms, but also in many cases it is difficult 

 or impossible to decide just where one species ends and the next 

 begins. That is, the most divergent individuals within a given 

 species frequently approach those of a closely similar species. There 

 are intergrades. (Fig. 236.) 



Furthermore it is found that species themselves can be naturally 

 arranged in more comprehensive groups to which the name genus 

 is applied. For example, the common Gray Squirrel represents the 

 species carolinensis, and the Red Squirrel, the species hudsonicus. 

 Both are obviously Squirrels, and therefore both species are grouped 

 under the genus Sciurus. Accordingly, each animal is given a name 

 composed of two words: the first, generic and the second, specific. 

 The Gray Squirrel is Sciurus carolinensis and the Red Squirrel is 

 Sciurus hudsonicus. Thus to give a scientific name to an animal or 

 plant is really to classify it, because the first word of the name indi- 

 cates that it possesses some fundamental characteristics in common 

 with the other species of the genus — in fact, is more like them than 

 it is like any other group of organisms. 



But again, the members of the genus Sciurus have many char- 

 acteristics in common with other animals which obviously are not 

 true squirrels. The Chipmunks or Ground Squirrels, for instance, 

 differ not only in certain obvious features, but in the possession of 

 internal cheek pouches, etc. This dissimilarity and similarity is 

 expressed by placing them in a different genus, Tamias, but in the 

 same family, Sciuridae. The familiar eastern Chipmunk is Tamias 

 striatus. 



.Moreover, while the Beaver (Castor americana) differs still more 

 from the Squirrels than do the Chipmunks, and therefore is placed 

 in a distinct family, the Castor idae, it nevertheless agrees with both 

 in many fundamental ways so that it is placed in the order Ro~ 

 dentia, which also includes the Squirrels and Chipmunks, as well as 

 many other families and genera. Other orders, such as the Ungulata 

 (Horses, Cattle, etc.), the Carnivora (Cats, Dogs, Bears, etc.), and 

 the Primates (Monkeys, Apes, etc.), while they differ widely from 



