1911 Encyclopædia Britannica/Species

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SPECIES, a term, in its general and once familiar significance, applied indiscriminately to animate and inanimate objects and to abstract conceptions or ideas, as denoting a particular phase, or sort, in which anything might appear. In logic it came to be used as the translation of the Gr. εἶδος, and meant a number of individuals having common characters peculiar to them, and so forming a group which with other groups were included in a higher group. The application of the term was purely relative, for the higher group itself might be one of the "species," or modes of a still higher group. In medicine it was used for the constituents of a prescription. In algebra it denoted the characters which represented quantities in an equation.

Early writers on natural history used the term in its vague logical sense without limiting it to a special category in the hierarchy of classification. To John Ray, the famous English naturalist, the credit is generally given of first making species a definite term in zoology and botany, but Ray owed much of his classification to Kaspar or Gaspard Bauhin (1550-1624), profressor of Greek and of Anatomy and Botany at Basel, and much of his clear definition of terms to an unpublished MS. of Joachim Jung of Hamburg (1587-1657). Sir W. T. Thisleton Dyer (Edinburgh Review, 1902, p. 370) thinks that Ray's use of the word may be traced to the last-mentioned authors. It is clear, however, that through Ray's work in the 17th century the common biological application of species became fixed much in its modern form, as denoting a group of animals or plants capable of interbreeding, and although not necessarily quite identical, with marked common characters. Working on these lines, and attaching special importance to common descent, naturalists applied the term with more and more precision, until Linnaeus, in his Philosophia botanica, gave the aphorism, "species tot sunt diversae, quot diversae formae ab initio sunt creatae" - "just so many species are to be reckoned as there were forms created at the beginning." Linnaeus' invention of binomial nomenclature for designating species served systematic biology admirably, but at the same time, by attaching preponderating importance to a particular grade in classification, crystallized the doctrine of fixity. The lower grades in classification such as sub-species and varieties on the one hand, and the higher grades on the other, such as genera and families, were admitted to be human conceptions imposed on the living world, but species were concrete, objective existences to be discovered and named. G. L. L. Buffon and J. P. B. Lamarck practically conceded the objective existence of species in arguing that they might be modified by external conditions, and G. L. Cuvier proclaimed their fixity without reserve. Charles Darwin found the conception of species so definite and fixed that he chose for the title of his great book (1859) the words On the Origin of Species by Means of Natural Selection, although his exposition of evolution applied equally to every grade in classification. E. B. Poulton, in an admirable discussion of contemporary views regarding species (presidential address to the Entomological Society of London 1904), has shown that Darwin did not believe in the objective existence of species, not only because he was led to discard the hypothesis of special creation as the explanation of the polymorphism of life, but because in practice as a working systematist he could neither find for himself nor ascertain from other systematists any settled criteria by which a group of specimens could be elevated into a genus, accepted as a species, or regarded as a variety.

The vast advance in knowledge of the existing forms of living things that has been acquired and recorded since 1859 has accentuated the difficulty of finding any morphological criteria for species. A few writers have insisted that they are discontinuous, and that real gaps exist between them. Equally great gaps, however, may exist between males and females, between climatic phases or summer and winter forms. The attempt to find a physiological criterion has similarly failed; many forms that have been universally accepted as true species produce fertile hybrids (see Hybridism). In modern practice (see Zoological Nomenclature) systematists no longer regard species as more than as an artificial rank in classification, to be applied chiefly for reasons of convenience, so that the word is reverting to its older logical significance. The word "species" now signifies a grade or rank in classification 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, and of which the known individuals, if they differ amongst themselves, differ less markedly than they do from those outside the species, or, if differing markedly, are linked by intermediate forms. It is to be noted that the individuals may themselves be judged to fall into groups of minor rank, known as sub-species or local varieties, but such subordinate assemblages are elevated to specific rank, if they appear not to intergrade so as to form a linked. species, whilst on the other hand assemblages judged to be species are merged, or degraded to sub-species, if they are found to intergrade by discoveries of linking forms. A species, in short, is a subjective conception, and some writers, as for instance E. Ray Lankester, have urged that the word is so firmly asssociated with historical implications of fixity which are now incongruous with its application, that it ought to be discarded from scientific nomenclature.

In technical biology each species is designated by two words, one for the genus, printed with an initial capital, and one for the particular species, printed without an initial capital in Zoology, whilst in Botany the habit once common to both subjects is retained, and the specific name if derived from a proper name is printed with a capital. The two words are printed in italics, and may be followed by the name of the author who first described the species. Thus "Canis vulpes Linnaeus" is the specific designation of the common fox, Canis being the generic term common to dogs, wolves and so forth, and vulpes indicating the particular species, whilst the attached author's name indicates that Linnaeus first named the species in question.

(P. C. M.)