INTRODUCTION xi 



This diagram shows how the subdivision passes from the larger to the smaller group 

 until finally a group is reached that contains only a single, particular kind of Mammal, and 

 this group we call a species. Of course it is obvious that each of the larger groups may have 

 more than one of the lower groups. Every Mammal has a similar place in the scheme, but 

 always its relationships to any other Mammal are shown by the fact that the two will be 

 included in some common group, if the groups are followed back far enough. 



Order Suborder Family Genus Species 



pennsylvanicus (Eastern Meadow 

 / Mouse) 



Microtus 



/ 

 Muridae 



/ 

 Simplicidentata 



/ 

 Rodentia 



■\ 



Duplicidentata 



\ 



Leporidae 



Lepus 



americanus (Varying Hare) 



Thus we see that the Eastern Meadow Mouse and the Varying Hare can claim no closer 

 relationship than that expressed by inclusion in the same order, a rather distant relation- 

 ship. The higher the rank of the first including subdivision, the more remote the imme- 

 diate relationship of any two Mammals. In common practice the sub-groups, like the sub- 

 family, sub-genus, and sub-species are often omitted, although with the knowledge that these 

 groups exist, the full divisions themselves expressing all the ord'nar}- needs of relationship. 



SCIENTIFIC NAMES 



For convenience sake and to show the authority for a name, it is customary to place 

 after a scientific name the name of the man who first described the animal. Thus, the scien- 

 tific name for the Eastern Meadow Mouse is written Microtus pennsylvanicus (Ord). This 

 means that Ord first gave a name to the animal we know as the Eastern Meadow Mouse; 

 while the parenthesis about his name indicates that the name has been somewhat changed 

 by some later authority from its original form. Ord called the animal Mus pennsylvanica 

 not knowing that the name Mus was first used for a different animal, an Old World rat, and 

 consequently could not be used for the American animal too without creating confusion; 

 therefore, a later worker straightened out the confusion and put parentheses about Ord's 

 name to indicate the change. No parentheses means that nothing has been changed, and 

 that the name stands as its author first wrote it. 



EVOLUTION .\ CONSTANT FACTOR 



The real necessity of a classification arises from the plastic nature of animal organisms. 

 If the Mammal were not the plaything of its environment, we should not have a multitude 

 of different mammals, and consequently we should not be at a loss to find names and 



