There are animals so like one another that they are given the same name. Such animals are ranked in the same _species_. Animals which differ so much that they have to be referred to different species, but which notwithstanding agree in the majority of their characters, especially the most important ones, are placed in the same _genus_. Hare and rabbit, or horse and donkey, are reckoned as different species of the same genus. Genera resembling one another are united into a _family_; thus, the pine marten and the beech or stone marten both belong to the _Marten genus_ (Martes), while the weasel and stoat are different species of the _Weasel genus_ (Mustela); but these two genera are so similar that they are both placed in the same family, _i.e._ the _Weasel family_ (Mustelidæ). Nearly related families together build up an _order_. Thus, the Weasel family, Dog family, Cat family, etc., collectively constitute the _order of Carnivora_, characterized, speaking generally, by the same kind of teeth, claws, habits, and food. Several related orders are united into a _class_. Thus, for example, carnivorous animals (Carnivora), ruminating animals (Ruminantia), gnawing animals (Rodentia), etc., constitute different orders of the _class of Sucklers_ (Mammalia); while birds of prey (Raptores), pigeons (Gyrantes), and poultry (Rasores), are included in a second class, that of _Birds_ (Aves). But both Birds and Mammals have a skeleton, of which the chief support is the backbone; on this account they are placed in a larger subdivision, the _sub-kingdom_ of Backboned animals (Vertebrata); while snails are grouped under the sub-kingdom of _Molluscs_, millipedes and centipedes under that of _Jointed-limbed animals_ (Arthropods). In this way the animal kingdom is divided into _sub-kingdoms_, the sub-kingdoms into _classes_, the classes into _orders_, the orders into _families_, the families into _genera_, and the genera into _species_. Animals of the same species which differ from one another in more or less constant characters, belong to different _races_ (domestic or geographical races). There are many species of animals the external features of which are well known to ordinary folk, and which therefore possess a definite English name, but a much larger number, of the smaller forms especially, have no English name. It is, therefore, necessary to devise new names for these species. The English names, however, are liable to cause great confusion, since in different districts the same name is often applied to widely different animals. Besides this, distinct names have usually been given to successive stages in the life history of the same form: “wireworms,” for example, are the young state of the “click beetle.”