IT IS a curious fact that the species, although the pivotal conception in systematic biology, has never been defined with precision in terms upon which the great majority of biologists have been willing to agree. To John Ray the species seemed somewhat more variable, somewhat less fixed, than to Linmaeus, and every generation of biologists since, to a g,reater or lesser degree, has aired its differences of opinion in our journals. The original goal of the great systematists who initiated the classification of animals was to provide a system of hierarchical categories, based upon morphological similarities and differences, which would comprise a convenient catalogue of extant organisms. To these early workers the species appeared as a static and permanent group, for the concept of evolution was still to be developed. But the truth of evolution shone through the system of classification which they invented. Their work stimulated evolutionary speculation and later, when Lamarck, Darwin, and others had formulated tenable theories of descent, the systematists played a very active part in confirming the new concepts. In this process the systematist accepted a new labor-that of arranging his categories in such a way that they would reflect the actual course of evolution. As attempts were made to work fossil forms into the classificatory structure new problems arose, but the endeavour has persisted and a reasonably satisfactory scheme has been evolved. The fact is that the systematic structure envisioned by the earliest workers was admirably adapted to the new purpose in many ways and was from the outset arranged to a considerable extent along the lines of a natural classification. It was evident from the first that if there were, actually, an evolutionary process the species must be a temporary phenomenon rather than a fixed category, that new species must appear as old ones vanish or differentiate into several types. Darwin's choice of a title is sufficient evidence of the early appreciation of this fact. It was generally assumed, however, that the species changes on a time scale so vast that for all practical purposes it could be treated as if it were unchanging and permanent. In more recent years, however, the discovery of mutations and other kinds of enduring changes of the germ plasm, and to an even greater extent the establishment of the field of population genetics, provided an insight into evolutionary mechanics. It became evident that the species, along with all of the other categories of the natural classification, must take its place as a natural unit-as some particular point in the evolutionary process. Accordingly, new conceptions of the species began to appear in conflict with the traditional conceptions of the early systematists. Past developments in the field of systematics and our discussions of the species category have tended to culminate in three more or less overlapping, more or less distinctive, conceptions of the species. Within these three schools of thought there are nuances of interpretation and minor areas of disagreement the discussion of which is beyond the scope of this paper. Let us content ourselves with examining some of the basic tenets of the three principal viewpoints with the idea of determining something of the potentialities and limitations of each, and with the hope of finding some common ground upon which all can meet. It is not easy to choose among the many excellent statements of the various points of view, The particular ones which the writer has chosen seem to him to embody,