THE ORIGIN OF SPECIES 349 



Again, it is found that species themselves can be naturally ar- 

 ranged 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 there- 

 fore both species are grouped under the genus Sdurus. Ac- 

 cordingly, each animal is given a name composed of two 

 words: the first, generic and the second, specific. The Gray 

 Squirrel is Sdurus 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 indicates that it possesses some fundamental char- 

 acteristics 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 

 characteristics in common with other animals which obvi- 

 ously 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, 

 Sduridae. 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 Castoridae, it 

 nevertheless agrees with both in many fundamental ways, so 

 that it is placed in the ORDER Rodentia, which also includes 

 the Squirrels and Chipmunks, as well as many other families 

 and genera. Other orders, such as the Ungulata (Horses, 

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

 while they differ widely from the Rodents, still agree with 



