CATEGORIES OF CLASSIFICATION. The number of known species of living animals is perhaps 350.000, to which must be added over 50.000 fos sil described species. making over 400.000 in all. To contain all these forms it is necessary to pro vide a complicated system of categories of vary ing ranks. We recognize the fact that, in beast as well as in man, no two individuals are exact ly alike. But just as the various members of the B. Smith family more nearly resemble each other than they do the Jones or Brown families, so certain other animals, while showing slight individual differences, possess so many common, eonstant qualities that we put them in the same species. Likewise, several species have certain in common, while they differ in other respects so markedly from all other that we class them together in one genus. In the same way, a number of related genera fall nat urally into the same Speriec.—A term applied in biology to the unit of elassitieation—that is, the lowest group that receives a name. (For the exreptional usage in respect to 'varieties.' see below.) Al though the term ',peeks' is almost universally employed by biologists to-day, a precise tion cannot he applied to it; for there is abso lutely no criterion by which a species may he distinguished from a variety, on the one hand, or a genus on the other. Whether a lot of or ganisms under consideration showing certain differences shall be considered two varieties, two species, or two genera, depends upon the per sonal characteristics of the classifier. If he be conservative, and has studied extensively the systems of the past, he will draw his ideals from them, and call those groups 'species' which show about that difference accorded to other related species by equally conservative classifiers in the past. If he be radical, he will disregard the ideals of his predecessors, and, according to his personal bent, 'lump' the diverse forms into one species, or possibly into one variety, or 'split' them into distinct species. indeed, a radical may take occasion to 'revise' the grouping of a conservative in accordance with his peculiar ideals—in consequence of which, numbers of spe cies may disappear by inclusion into old ones. or new ones may be added by division of old ones: or, finally, the grouping may be thorough ly reorganized or disarranged. At the present time naturalists have hit upon no method of avoiding this intolerable condition of affairs, which is rapidly bringing chaos into what was supposed to be a 'system.' A century ago. be fore so extensive collections had been made and studies were so critical, it was generally believed that 'species' were perfectly distinct as well as immutable things. Even many of the scientific workers regarded the mythology of the first chapters of Genesis as a scientific record, and admitted that there were only so many 'species' as were created in the beginning: that each species received its name from Adam, and that one pair of each (except parthenogenetic spe cies) was preserved in the Ark, to become the progenitors (or progenitor) of all the members of the species which have existed since. The
worthlessness, for scientific purposes, of this literary account of the creation appears when we try to enumerate the 'species' that were thus created. We then realize that 'species' are not natural phenomena, but human devices of con venience, like the ward-boundaries of a city. The boundary lines between species may. in some eases, be in part determined by natural phenomena, as ward-boundaries may be limited by a stream or a bluff.
The history of the ideas entertained concern ing species is an epitome of the most profound biological thought. In the development of any large idea, three stages may be recognized—first, the speculative suggestion of it; second, the clear statement of it as a working hypothesis; and, third, the demonstration. Such has been the history of the development of the modern idea of species as part of a continuous stream of life. The early Greeks were not troubled with the idea of species; they sought only a satis factory speculative account of the origin of or ganisms. The early Christians interpreted the .Alosaie' account of the creation liberally. regard ing the creation not so much as the forming of completed organisms as the forming of the seed out of which they were to arise. During the Tliddle Ages, however, and especially from the middle of the sixteenth to the middle of the nineteenth century, the doctrine of 'special crea tion' was universally taught by the Church. ;Hilton's epics popularized the idea of creation ready made, with adult animals of the different species; and the rigid conceptions of species which the early botanists and zoiilogists bad in vented seemed to support the teachings of the churchmen. With the founding of universities in the twelfth and thirteenth centuries, science began to awaken, and one of the first products of the Renaissance was the natural philosophers. Such men as Francis Bacon, Descartes, Leibnitz, and Spinoza did not limit their ideas to the teachings of the Church; they taught the neces sary principle of continuity, to which the natural development of 'species was a necessary corol lary. The more speculative naturalists, or phi losopher-naturalists, next applied the doctrine of continuity, as an hypothesis, to the facts of organisms. Such were 'Bonnet, who recognized that all life was continuous, and Men, who conceived that all organisms have developed out of a primitive slime. Finally came the great naturalists, putting forth the idea of the muta tion of species tentatively at first, then with greater vigor, until Darwin came, at the fullness of time, to precipitate the revolution. See EVO LUTION ; DARWIN ; LAMARCK.