PRINCIPLES OF CLASSIFICATION. 323 



resemblance or difference are presented, but always with a mani- 

 fest tendency to compose groups or associations of resembling 

 species, groups the more numerous and apparently the less 

 definite in proportion to the number and the nearness of the- 

 points of resemblance. These various associations the naturalist 

 endeavors to express, as far as is necessary or practicable, by a 

 series of generalizations, the lower or particular included in the 

 higher or more comprehensive. All kinds of differences are 

 taken into account, but only the most constant and definite ones 

 are relied on for characters, i. e. distinguishing marks. Linnaeus 

 and the naturalists of his day used names for only three grades 

 of association, or groups superior to species, viz. the Genus, the 

 Order, and the Glass ; and these are still the principal members 

 of classification. 



642. Genera (plural of Genus) are the more particular or 

 special groups of related species. They are groups of species 

 which are much alike in all or most respects, which are con- 

 structed, so to say, upon the same particular model, with only 

 circumstantial differences in the details. They are not neces- 

 sarily nor generally the lowest definable groups of species, but 

 *re the lowest most clearly definable groups which the botanist 

 recognizes and accounts worthy to bear the generic name ; for 

 the name of the genus with that of the species added to it is the 

 scientific appellation of the plant or animal. Constituted as the 

 vegetable and animal kingdoms are, the recognition of genera, or 

 groups of kindred species, is as natural an operation of the mind 

 as is the conception of species from the association of like indi- 

 viduals. This is because many genera are so strongly marked, 

 at least so far as ordinary observation extends. Every one 

 knows the Rose genus, composed of the various species of Roses 

 and Sweetbriers ; the Bramble genus, comprising Raspberries, 

 Blackberries, &c., is popularly distinguished to a certain extent ; 

 the Oak genus is distinguished from the Chestnut and the Beech 

 genus ; each is a group of species whose mutual resemblance is 

 greater than that of any one of them to any other plants. The 

 number of species in such a group is immaterial, and in fact is 

 very diverse. A genus may be represented by a single known 

 species, when its peculiarities are equivalent in degree to those 

 which characterize other genera. This case often occurs ; al- 

 though, if this were universally so, genus and species would be 

 equivalent terms. If only one species of Oak were known, the 

 Oak genus would have been as explicitly discerned as it is now 

 that the species amount to three hundred ; and better defined, 

 for now there are forms quite intermediate between Oak and 



