SPECIES. The nature of species is a question of considerable importance in general biology and the species is regularly employed as a classificatory unit in systematic botany, zoology and miner alogy. The word species has, however, an important history out side those sciences. It was the Latin equivalent of the Greek ethos ihict . This meant primarily shape or visible form, but was extended to cover the sensible character of things generally, and also the intelligible. For the former use compare the term "sensible species" or characters in things perceived, because re produced in the organs of sense. To the supposition that we see these reproductions is partly due the doctrine that the mind per ceives directly only its own ideas. So Locke says (Essay Con cerning Human Understanding, Introduction, § 8) that he uses the word idea to express whatever is meant by phantasm, notion, "species." As intelligible character, ethos or species was the com mon essence of individuals or instances, as we say, of the same kind. As such it was an object of thought, not of sense ; universal, not individual : and, unlike its instances, exempt from generation and decay. Its relation to its instances (which may be compared to that of a law to the events exemplifying it) was a subject of dispute both in Greek and scholastic philosophy. There were also disputes about the relation of species to their common genus, of which they are determinations (so that Aristotle compared the relation of genus to species with that of matter to form), and to the properties connected and accidents conjoined with their spe cific nature in instances of a species. Boetius' Latin translation of the lsagoge (Introduction) written by Porphyry (born A.D. 232) to the Categories of (or ascribed to) Aristotle made these questions familiar long before the revival of Aristotelian learning in the West. In particular it was disputed whether species and genera were anything real independently of our minds, or only common names, or else notions of ours, the products of our classi fying activity. A settlement of these questions is not necessary to the work of the systematic biologist ; but they must be faced if we are really to understand what we mean by the evolution of species; for this evolution is a process of which no persisting material thing is the subject. (H. W. B. J.) In systematic zoology and botany the term at present denotes a group of individual animals or plants which is allotted a more or less definite place in the hierarchy of classification between the genus and the variety. A species may be divisible into several varieties or sub-species and a genus may contain one or more species. As the differences between varieties are often very difficult to recognize, it is the species that to the untrained observer usually seem to represent the simplest distinct assemblages or "kinds" in the animal and plant kingdoms. Thus the missel thrush (Turdus viscivorus) can be recognized as distinct from the song thrush (T. philomelus) with greater or less ease, but their varieties are only regularly perceptible after intensive study. Aristotle was the first to use the term "species" in natural history and it was employed by him in the logical sense outlined above, i.e., as denoting a lower division of a higher class. Groups which rank as species in certain of his larger classes are them selves subdivided into species. Thus the crabs and squills are treated as species of Crustacea and the crabs are again sub divided into species. For many centuries after Aristotle the species-concept was employed in this manner. It is true that
differences between closely related organisms, such as the wild and cultivated races of plants, were appreciated by the Greek naturalists and herbalists (e.g., Hippo of Rhegium, 423 B.c.), but the early naturalists were not concerned with the production of a "natural" scheme of classification, and names were bestowed upon animals and plants without any reference to such a system. But it would not be true to say that the early naturalists and pharmacologists had no knowledge of the diversity of nature. Many of them were fully acquainted with their local floras; students such as Dioscorides were able to enumerate as many as I,000 kinds of plants. Our modern classification is based on the principle of relationship and it was probably not until botanists began to cultivate plants experimentally and to collate their re sults with herbarium collections that the importance of relation ship was grasped. Progress upon these lines was made by the Swiss botanist, Caspar Bauhin (1550-1624). Towards the end of the 17th century Martin Lister, an English naturalist, who founded the special study of conchology, employed the species concept in his Historiae Conchyliorum (1685) very much as it is used now. Certain groups recognizable as genera in modern classi fication were subdivided by him into subordinate groups (not actually called species) which were according to the custom of the time designated by their distinctive characters. It is, how ever, to John Ray, another English naturalist, and a contemporary of Lister, that we owe the first serious discussion as to the status and nature of species and the use of the criterion of mutual fertility as a test of specific identity. The position of the species as the lowest member of a classificatory system was exactly defined by C. N. Lang (167o-174o). At this period there seems to have been no attempt to delimit species one from another with any strictness of definition. A notable feature was a growing in terest in their origin, and, as Bateson (1913) has pointed out, there was a distinct tendency to believe that they could arise spon taneously. This belief was, however, checked by the influence of C. Linne (see LINNAEUS). The celebrated Swedish naturalist in his Philosophic Botanica (175i, §157) made use of the following definition : species tot numeramus quot diversae formae in prin cipio sunt creatae, which may be freely rendered "our classifica tion contains just so many species as there were different forms created in the beginning." This famous aphorism is accompanied in the text by a quotation from an earlier work, the Classes Plantar= which leaves no ambiguity as to the meaning of in principio—species tot sunt quot diversas formas ab initio pro duxit infinitum Ens ("there are as many species as there were different forms produced in the beginning by the Infinite Being"). Linne thus regarded species as archetypal—as definite entities im mutable and fixed in the mould of the original creation. The quo tation proceeds : quae formae secundum generationis inditas leges produxere plures at sibi semper similes ("these forms according to the laws of reproduction have produced more [sc. individuals] always like themselves"). It must be observed that the aphorism and its accompanying quotation, when read in conjunction with the definition of variety, (I) define the origin of species and (2) propose as criteria of specific status the test of breeding true to type and the structural uniformity of the constituent individuals.