Species. This is a term which it is very difficult to define with precision. The word itself means a look, an appearance, a kind; and in common usage things that look the same are said to be of the same species. With more definiteness naturalists speak of a mineral species, and of a species of plants or animals. It is with this last usage that we are here concerned.
In classifying plants or animals we form conceptions of various degrees of comprehensiveness (see BIOLOGY), and for these we use a series of terms, such as class, order, family, genus, species, variety. The need for precision is that every one may know exactly what is meant when any individual or group of individuals is named. In the ordinary system of classification a species is a group of individuals which closely resemble one another, and the species is usually subordinated to a genus—a wider group of similar, but less closely similar, forms—and is superior to a variety, of which there may be several in a species. Thus we group the lions as a species (Felis leo) of the genus Felis, in the family Felidæ, order Carnivora, class Mammalia, and call the tigers, leopards, cats, and the like other species of the same genus Felis. As no one could confuse lion, tiger, and leopard, for the peculiarities of each are well marked, it may be wondered what difficulty there is in defining species.
Let us consider the matter practically. We observe our fellow-men; we see that they differ in many ways from one another, in stature, in features, in complexion, in colour of eyes and hair, and so on; but we do not think of speaking of a red-haired or a blue-eyed species of man. We should as soon think of saying that the red-haired or blue-eyed child in a family was of a different species from its brothers and sisters or from its parents, which would be absurd. We at once agree with the systematist when he says that the term species should not be given to a group of individuals which are distinguished from other groups by no greater differences than distinguish members of a family, and when he says that the characters of a species must have some constancy from generation to generation, which is not of course the case with red hair or blue eyes. This is a common-sense way of limiting the term, but it leaves many difficulties untouched. It is not readily applied to extinct species, of whose generations and individual variations we cannot know much; nor has it been applied to a vast number of forms recorded as species sometimes on the strength of single specimens, and often without any knowledge of their generations.
But, again, we observe men with much greater peculiarities than red hair or blue eyes; we contrast Britons and Chinamen, Lapps and Negroes, and we wonder if these really belong to the same species. Here, however, the systematist reminds us that the members of a species are fertile inter se, which cannot be denied of the different races of mankind. But a little knowledge is enough to keep us from attaching very much weight to this distinction, since both among plants and animals there are many cases of fertile hybrids between different species. We can no longer talk as if the mule were the only known hybrid. See HYBRID.
Or if we turn to the systematic treatises which classify plants and animals, and compare half a dozen of them, we find ample evidence of the elasticity of the conception of species. Quot homines tot sententie. Thus, as Haeckel notices, one botanist enumerates 300 German species of the common Composite, Hieracium; another reduces them to 106, another to 52, another to about a score! Bechstein said that there were 367 species of birds in Germany, but according to Reichenbach there are 379, according to Meyer and Wolf 406, according to Pastor Brehm 900! But Haeckel himself supplies the best example, for in his important monograph on Calcareous Sponges he admits that as the naturalist likes to look at the problem there are 3 species, or 21, or 289, or 591! We are told that species are groups of individuals agreeing in essential characters which remain constant from generation to generation. But what are essential characters? and how much constancy is demonstrable?
We cannot forget, for instance, how one species may include forms so very different as Shetland pony, hunter, and dray horse; or as poodle, bulldog, and greyhound; or as carrier, pouter, and fantail; or as cabbage, cauliflower, and Brussels sprouts. Yet it is probable that in each of these four groups the diverse forms have been derived from the same ancestral wild species, and in each case the diverse forms are connected by intermediate stages.
In short, the fact is that there is no such thing as species. Individuals are real; but a species is a subjective conception. It is based on structural resemblances between individuals, and the degree of importance attached to these depends, as we have seen, on the mind of the observer, or is, in other words, entirely relative.
But while there can be no perfectly strict definition of a species in terms of morphology, it may be asked, where is there a possible one in terms of physiology, in terms of functional peculiarities about which there can be no dispute? At one time it almost seemed that there might be some solution in terms of fertility and infertility. But, although this distinction is certainly helpful and very important, it also breaks down. It is not of course doubtful that species have physiological peculiarities; lions differ from tigers in habit and chemical composition as well as in form and structure; but every individual has also its peculiarities—chemical as well as personal; the difficulty is to decide when these peculiarities are important enough to make it useful to give a precise name to their possessors.
As to the practical question of determining species it should be borne in mind that the differences between one form and another are often very marked, and that the gap between related forms is not always bridged by any unbroken series. The New Zealand lizards (Hatteria punctata) not only form a species, but are the sole living representatives of an order, or of a class; and the same may be said of the lancelet and other forms. It is with those organisms of which there are very many more or less different forms that there is real difficulty—with Bacteria, Algae, Protozoa, Sponges, Crustaceans, Insects, Fishes, Birds, and so on. In such cases the naturalist who admits that species is but a relative conception, and who, as an evolutionist, recognises the variability of species and the links of relationship which bind form to form, cannot do more than try to make sure that the peculiarities on account of which he gives a new name to any group of creatures are greater than those which distinguish the members of a family of these, are relatively constant from generation to generation, and are associated with reproductive variations which tend to restrict the range of mutual fertility to the members of the proposed new species. Unfortunately, however, species often are and sometimes must be established for single specimens, without any knowledge of their reproduction and generations, without any statistics of their variations, or careful comparison of these with those of related forms. Where the form in question is conspicuously unique the erection of a new species is of course readily justified.
History.—The history of the biological conception of species can be rapidly sketched. We need not go farther back than John Ray (1628–1705), who gave to the term species that meaning which it bore until evolutionary conceptions prevailed. Linnaeus adopted the usage defined by Ray, and by introducing the 'binomial nomenclature' made it more serviceable. He believed in real species, and said: 'Species tot sunt diversae, quot diversas formas ab initio creavit infinitum ens,' though some hesitancy in regard to this is shown when he elsewhere expresses the idea that all the species of a genus ab initio unam constituerunt speciem. Although Lamarck in 1809 declared species to be artificial conceptions, the Linnean idea prevailed—even in Agassiz' Essay on Classification (1859)—until Darwin and his fellow-workers modified this among many other conceptions by establishing the doctrine of evolution, 'by which,' as Ray Lankester says, 'universal opinion has been brought to the position that species, as well as genera, orders, and classes, are the subjective expressions of a vast ramifying pedigree in which the only objective existences are individuals.'
See BIOLOGY, BOTANY, DARWINIAN THEORY, EVOLUTION, GENUS, VARIATION, ZOOLOGY. See also Darwin, Origin of Species (1859); Haeckel, Generelle Morphologie (1866); Natürliche Schöpfungsgeschichte (1868; 8th ed. 1889; trans. Natural History of Creation, Lond. 1879); Die Kalkschwämme (1872); Spencer, Principles of Biology (1864–66); Wallace, Darwinism (1889).—The species of Logic is originally suggested by Natural Species, and may be defined as a group of individuals agreeing in some common character and known by a common name; two or more species constituting a genus. The relation of species to the other logical elements of classification are treated at PREDICABLES and GENERALISATION. The great controversies as to the real existence of species and other universals are sketched at NOMINALISM.