24 GENERAL ZOOLOGY 



individuals composing a species are never identical, and the 

 individual differences which constitute these variations make it 

 difficult to define the limits of a species. Such gradations among 

 or between species are exactly what would be expected on an 

 evolution basis; while, on the other hand, variations have been a 

 stumbling block to upholders of the doctrine of special creation, 

 who with Linnaeus believed that species did not develop gradu- 

 ally but were made outright, and that all species were therefore 

 fixed. At present, the idea that species are fixed is no longer 

 tenable because of the vagueness of the boundaries between 

 them, but the employment of the term "species" is useful in 

 describing the basic unit in the system of classification. 



Scheme of Classification. — Species are arranged in groups of 

 higher order called genera. A number of similar species consti- 

 tutes a genus in much the same way that a number of similar 

 individuals makes up a species. The characteristics which distin- 

 guish one genus from another are more deep-seated and funda- 

 mental than those that distinguish species. Related genera, in 

 turn, are combined into families, families into orders, orders into 

 classes, and classes into phyla. The phylum, then, is the largest 

 group in either the plant or animal kingdom. Each group is 

 sometimes made up of subgroups, such as subphylum, subclass, 

 etc. ; and occasionally entirely new terms are introduced to meet 

 the needs of classification. The custom of employing Latin or 

 Greek derivatives in the technical naming of these groups is now 

 universally followed and makes for clearness in identification. 



Binomial System. — Every known kind of animal and plant has 

 a scientific name, consisting of the name of its genus, capitalized, 

 followed by the specific name in small letters. Thus, the name of 

 the house cat is Felis domestica; a common species of frog (Leopard 

 frog) Rana pipiens; Man, Homo sapiens. This method of naming 

 was formulated by Linnaeus and is known as the binomial 

 system of nomenclature. It is also customary to follow the 

 scientific name with name of the author who originally proposed 



it. 



Trinomial System. — It sometimes happens that the difference 

 between two kinds of animals is so slight as to scarcely warrant 

 placing them in separate species, and yet requires some practical 

 method of classification. In such cases the intraspecies dis- 

 tinction is made by adding a subspecies name to the name of the 



