V. The Affinities of Animals, etc. 55 



forms constitute a special yariety ; for, as a rule, they differ in some 

 respects from the European. On the other hand, if a group of 

 individuals found in a larger or smaller tract of land or water, differs 

 in certain characters without exception, from similar individuals 

 of another locality, then it forms a distinct but nearly related 

 species. The Wapiti Deer (Oervus canadensis) of North America, for 

 instance, is a different species from the European Bed-Deer, but is 

 nearly allied to it. So also the North American Beaver is a different 

 species from the nearly related European form, since some individuals 

 of the first display peculiar characters (specially in the skull) which are 

 never present in the European, and conversely. Often it is very 

 difficult to determine whether a group of animals forms a variety or 

 an independent species ; it is difficult, in proportion as the particular 

 variations are constant or not, as they are present occasionally 

 (variety), or without exception (species), and of this it is not easy to be 

 perfectly certain. Practically, a group of individuals is regarded as a 

 definite species when transitional forms from it to another are 

 unknown, i.e., when no specimens exhibit even a partial oblitera- 

 tion of these characters, which separate them from a closely allied 

 species. So soon as such links are found, the group is recognised 

 as a variety of the other. There is no sharp limit in Nature between 

 a variety and a species. 



The different beliavioiu' of species and varieties in crossing also, as was 

 mentioned earlier, cannot be employed as an absolute specific test, for, altbougb 

 hybrids of different varieties are almost always perfectly fertile, this may 

 also be the case with hybrids of species (see above, p. 36). 



Similar species are grouped as a genus; e.g., the lion, tiger, 

 jaguar, cat, and others as species of one genus. Each genus of the 

 Animal Kingdom is designated, according to a fixed rule, by its Latin or 

 latinised name, which is always a single word ; e.g., the genus which 

 includes the animals just mentioned is called Felis. A species is 

 known by the name of the genus with a subjoined distinguishing 

 name ; the latter is usually an adjective, sometimes a substantive in 

 apposition to the name of the genus, or a substantive in the genitive 

 case ; the domestic cat, for instance, is called Felis domestica, the Uon, 

 Felis leo, the cochineal insect. Coccus cacti. Several similar genera 

 are collected together as a family. For example, the badger-genus 

 [Meles], the otter-genus [Lutra], the marten-genus {Mustela), and 

 others, all belong to the marten-family, Mustelidse ; the family name 

 is usually formed by appending the termination idse to the name 

 of one of the genera. Several families, again, are united to form an 

 order, the cat-family, marten-family, bear-family, etc., form the 

 order Carnivora. Orders, again, constitute classes, e.g., the 

 Carnivora and a number of other orders belong to the class Mammalia. 

 Classes form phyla, e.g., the classes Mammaha, Aves, Eeptilia, etc., 

 form the phylum Vertebrata. 



