The system of zoölogical classification which is still in use was largely the invention of the Swedish naturalist Linnæus, who published it shortly after the middle of the eighteenth century. As devised by Linnæus, the scheme was intended to express ideal relationships, whereas now it is employed to express real genetic affinities, so far as these can be ascertained. The Linnæan system is an organized hierarchy of groups, arranged in ascending order of comprehensiveness. In this scheme, what may be regarded as the unit is the _species_, a concept around which many battles have been waged and concerning which there is still much difference of opinion and usage. Originally a term in logic, it first received a definite meaning in Zoölogy and Botany from John Ray (1628-1705) who applied it to indicate a group of animals, or plants, with marked common characters and freely interbreeding. Linnæus, though not always consistent in his expressions on the subject, regarded species as objective realities, concrete and actual things, which it was the naturalist’s business to discover and name, and held that they were fixed entities which had been separately created. This belief in the fixity and objective reality of species was almost universally held, until the publication of Darwin’s “Origin of Species” (1859) converted the biological world to the evolutionary faith, which declares that the only objective reality among living things is the individual animal or plant. According to this modern conception, a species may be defined as signifying a “grade or rank assigned by systematists to an assemblage of organic forms which they judge to be more closely interrelated by common descent than they are related to forms judged to be outside the species” (P. Chalmers Mitchell). The technical name of a species, which is either in Latin, or in latinized form, is in two words, one of which designates the genus (see below) and the other the particular species of that genus, as, for example, _Equus caballus_, the species Horse, _E. przewalskii_, the Asiatic Wild Horse, _E. asinus_, the species Ass, etc. In order to identify a species, the genus to which it belongs must be stated, hence the term, _binomial system_ of nomenclature, which Linnæus introduced, becoming _trinomial_ when the name of a subspecies is added, a modern refinement on the older method. A very large species (_i.e._ one which is represented by great numbers of individuals), extending over a very large area, is often divisible into groups of minor rank, as _varieties_, _geographical races_ or _subspecies_. Taking the species as the unit in the scheme of classification, the varieties and subspecies may be considered as fractions.