For nearly a century after Linne's death the doctrine of the fixity of species was upheld by the majority of biologists. The effect of Darwin's exposition of the causes of evolution and the evidence he produced in favour of natural selection very naturally led to the rejection of the Linnaean doctrine. The implications of Darwin's theory plainly pointed to the conclusion that varieties are incipient species and that species are progressively developed by the selection and accumulation of individual varia tions from a parent stock, aided in some measure by isolation. The line between variety and species is often imperceptible and, in Darwin's words, the term species "is arbitrarily given" and "does not essentially differ [in meaning] from the term variety." He held that the only criterion by which we can decide whether a doubtful form is to be ranked as a species or as a variety is "the opinion of naturalists having sound judgment and wide experience." It had previously been urged that the sterility of hybrids resulting from certain crosses afforded a good general criterion of specific distinctness. Darwin showed that this cri terion could not be accepted, as the degree of sterility of various species when crossed is so different "that for all practical purposes it is most difficult to say when perfect fertility ends and sterility begins." It should be clearly understood that the acceptance of this opinion concerning species does not mean that the naturalist is unable to recognize such distinct and sharply defined groups as may actually occur in nature. What was abandoned was the belief in the special creation of species and the less definitely expressed but implicit conception of a criterion by which species could be regularly recognized as distinct, e.g., from varieties.
Since the publication of the Origin of Species up to the present time many zoologists and botanists have resolutely followed Dar win in regarding the criterion of specific status as an arbitrary matter having a purely systematic value and in believing that species cannot be regularly discriminated from other categories as a distinct and standardized evolutionary grade. On the other hand there have been many students who, influenced by advances in our knowledge of the laws of heredity, have been less disposed to deny to the species a special and peculiar status. Alexis Jordan and H. de Vries showed that some of the Linnaean species were not homogeneous in hereditary constitution but were composed of a number of subdivisions or "elementary species." According to de Vries these arise not by slow progressive selection, but by sudden discontinuous change (mutation). The origin of new forms, whether they be "species," "subspecies" or "races" is discussed in the article EVOLUTION. It may be stated, however, that, although mutation in de Vries' original sense has not been shown to be of general occurrence, new forms have arisen as the result of crossing. These are recorded among plants and it may be noted that they sometimes fulfil the ordinary criterion of species. A more exact knowledge of the mechanism and laws of heredity was developed in the years following the republication of G. Mendel's celebrated investigations in 19oo (see HEREDITY, MENDELISM). Experi mental pedigree culture and work such as that of Johannsen on Pure Lines (q.v.) yielded evidence of the same type of heredi tarily constant strains as were demonstrated by Jordan and de Vries. The species thus recovered some of its lost prestige, at least in so far as the existence of ultimate units seemed to be established. In 1913 Bateson could speak of "the universal pres ence of specificity" among animals and plants. Most biologists at the present time, however, are unwilling to admit that species can be discriminated from varieties by a universal criterion. Some points concerning variation and other matters may make the reason for this conclusion clearer and should at the same time indicate the difficulty encountered in trying to fit into rigid categories the variation that is found in nature. ( ) A distinction must be drawn between the species described in systematic zoology and botany, or known to the casual observer, and the "natural population." The first is represented by the usually limited number of individuals accessible to the naturalist or en countered by the observer and is only a sample of the natural population found in nature. Very often the species of the sys tematist are represented by a few specimens only and it is rarely that the distribution and variation of species in nature are known with any exactness. (2) When such natural population are studied
in detail it is often found that they are far from homogeneous. Not only do the individual organisms referred to a species differ somewhat among themselves in single characters, but also the combinations of characters which they exhibit are often very diverse. A form designated as a species may be split up into local races which differ in the extent to which they are differentiated one from another. (3) It is sometimes possible to recognize dis tinct and discontinuous groups ; but forms diagnosed as species are often found to grade by a series of intermediate forms into other groups and species thus connected by intermediates and having many characters in common are only recognizable by the fact that certain combinations of characters are more frequently found than others. The difficulty of defining species with exact ness has been shown with particular clearness in the study of cer tain fossil animals. The intensive study of series of such forms obtained from strata succeeding each other in regular sequence usually shows a gradual change of one form into another and sometimes the "lineages" or successive phases of individual char acters are so inextricably confused as to make it impossible to recognize species in the ordinary systematic sense. (4) Inter mediate forms may be of two kinds : "mid-intermediates," i.e., forms in which all the characters represent a blend of two more extreme types (as grey is intermediate between black and white), and forms which represent various combinations of some of the characters found in two divergent stocks. The former may owe their intermediate appearance to the local environment and both kinds of intermediacy may be due to the effects of hybrid crossing. (5) The rer:ognition of species by a morphological criterion is embarrassed by the occurrence of sexual dimorphism, i.e., the occurrence of strongly marked differences in structure or colour between the males and females of a single species. (6) The cri terion of specific distinctness usually adopted is that of structural difference. Evolutionary divergence, however, is manifest in many other attributes of animals and plants—in the products of their metabolism, their reactions, habits, distribution, etc. It is found that the various modes of distinctness—structural, physiological, habitudinal, etc.—are not very closely correlated. For example, forms which are morphologically distinct may be mutually fertile or have identical physiological properties or activities. Although it is customary to regard structure as the surest clue to relation ship it is nevertheless to be borne in mind that structural differ ences do not constitute the only type of divergence and that, while our classification aims at expressing genetic relationship, it does not necessarily express the functional diversity of animal and plant life. It is perhaps desirable to emphasize the purely arbitrary nature of the criterion of specific status. Although at any one time it is possible to recognize tolerably distinct assemblages of individuals on which it is convenient to confer specific rank, never theless the limits of such "species" are, when considered in the light of evolutionary history, incapable of exact definition. In systematic zoology and botany a species is designated by two names, the generic name, indicated by an initial capital letter, and the specific, which is always now printed without a capital in zoology, though in botany the initial letter is a capital, if derived from a proper name. Both names are printed in italics and may be followed by the name of the author who published the first description of the species. Thus Corvus corax Linne (or Linn.) is the specific designation of the raven. Owing to complications in troduced by certain inevitable errors the nomenclature of species and genera is subject to a code of international rules.
The first described specimen of a species is known as the "type" of the species ("co-type" and "paratype" being used for various grades of "types"), a term which must not be confused with "typical," i.e. average.
In mineralogy the classificatory system is not based upon descent and relationship. Certain fundamental kinds of minerals known as "species" are recognized and defined according to their chemical composition and crystalline form. The larger groups are quite arbitrary and depend upon the purpose for which the classification is to be used. Some of the species of minerals are clearly defined. (See MINERALOGY.)