Page:The New International Encyclopædia 1st ed. v. 04.djvu/928

* CLASSIFICATION OF ANIMALS. 818 CLASSIFICATION OF ANIMALS. hitt'ly HO ciitorion by which a spcties may be liistiiiguished from a variety, on the one hand, or a genus on the other. Whether a lot of or- ganisms imder consideration showing certain dificrences sliall 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 iheni, 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, "lunip' 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 Avorthlessness, 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 boimdary 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 blufl". 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 modem 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 'Mosaic' 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 whidi they were to arise. During the Middle Ages, however, and especially from the middle of the sixteenlh to the middle of the nineteenth century, the doctrine of 'special crea- tion' was universally taught by the Church. Milton's epics popularized the idea of creation ro'ady made, with adult animals of the different species ; and the rigid conceptions of species which the early botanists and zoologists had in- vented seemed to sujiport 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 liacon, Descartes, Leibnitz, and Spinoza did not limit their ideas to the teachings of the Church ; they taught the neces- sary ])rinciple of continuity, to which the natural development of 'sjiecies" was a necessary corol- lary. The more speculative naturalists, or phi- losopliernaturalists, 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 Oken, 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- LL'TIOX; D.VEWIN; L.M.iKCK. The view that species are wholly arbitrary groups is not accepted by all naturalists. De Vries especially lays stress on the point that species are objective. Every species, he be- lieves, is made up of a number of specific char- acters, each of which dejjends upon a certain chemical particle or aggregation of molecules, which perj)etuates itself and multiplies by self- division. So long as the descendant chemical particles remain like the parental ones, so long the species persists. These particles may, how- ever, suddenly change through some cause, not precisely known, but probably environmental; and with a change in one of these particles goes a change in the adult specific quality that it con- trols, and also of the 'correlated' qualities. The consequence is that a form with many new quali- ties suddenly arises, and tliis form we may call a new species. De Vries has studied (Die Miita- tionslheorie, KJOl) variable species in the field — for example, the evening primrose — and finds that new fonns, clearly marked off from the original, typical ones, are constantly arising. They are, from the beginning, distinct and fully formed species. There are, however, other theo- ries to account for that 'distinctness' or isola- tion which is not infrequently found. One is that species originated in isolated situations. A group of land animals, getting by acci- dent upon an island and varying normally, will in time come to be quite dissimilar from the forms on the mainland, becau.se, being isolated, any pe- culiarities that may crop out among them will not be swanified by intercrossing with the main m.ass of the s])Ocies. Thus, we find in the Gala- pagos Islands that eacli island has a peculiar species of lizard of a particular genus, and only one species occurs on each island. The same is true of certain sparrows on these islands. Even on islands nearer the mainland, like Nantucket or Fire Island, on the south side of Long Island, there are ])eeuliar forms — incipient species. It is the isolation which has permitted them to arise; even a less degree of isolation may per- mit a difference to grow up. Tlius. Darwin tells this story: ''The two flocks of Leicester sheep kept by Mr. Buckley and Mr. Burgess have been jiurely bred from the original stock of Mr. Bake-