122 AN INTRODUCTION TO ZOOLOGY. 



"classes" in another group, and that two "orders" 

 in one class may be very mucU more alike than two 

 " orders " in another class. In technical language, two 

 classes, or two orders, are not necessarily co-equal in 

 morphological value. It naturally follows that different 

 writers may take different views as to the importance 

 of any given group of animals, and of its differences 

 from other groups; and that group may accordingly , 

 appear in one classification as part of an order, while 

 in another classification it forms an order by itself. 

 Similar differences of opinion may exist regarding 

 families or genera, or, in fact, regarding any of 

 the divisions of a classification ; hence no two text- 

 books will be found to give the same classification. 

 Although this fact is very puzzling to the beginner, it 

 has its advantages ; for it prevents the learning of 

 a system of classification by parrot-like repetition, 

 and makes the student inquire into and remember the 

 facts on which classification is based. 



Each kind of animal receives, in systematic nomen- 

 clature, two Latin or Latinized names, of which the 

 first is the name of the genus, the second that of the 

 species. This method of naming is called the binary 

 nomenclature. It is often a very dilficult matter to 

 draw the line between nearly related kinds. The test 

 of identity of species is usually this, that individuals 

 that breed together freely, and produce perfectly 

 fertile offspring, belong to the same species, even if 

 they look rather different from each other. Such 

 differences in colour, size, shape, and habits as may 



