WHAT IS MEANT BY "A SPECIES"?

THOSE who take an interest in natural history must find it necessary to know what the naturalist means by "a species" of animal or plant. What does he mean when he says: "This is not the same species as that," or "This is a species closely allied to this other species," or "This is a new species"? What are the "species" concerning the origin of which Darwin propounded his great theory? There is really no English word which can be exactly used in place of the word "species." I often have to use the word when writing about plants or animals, and should like once for all to say what is meant by it. One might suppose that a "kind" is the same thing as a species. And so it often is; but, on the other hand, by the word "kind" we often mean a group including several species. For instance, we say the "cat-kind" or the "daisy-kind," meaning the "cat-like" animals or the "daisy-like" plants. The expression "the cat-kind" includes the common cat and the wild cat, and even leopards, lions, and tigers, each of which is a species of cat. And by the "daisy-kind" we understand a group including several species of daisies, such as the common daisy, the ox-eye daisy, the camomile daisy, the michaelmas daisy, and others. Hence we cannot translate species simply by the word "kind." "Kind" is the same word as "kin"—"a little more than kin and less than kind," runs Hamlet's bitter pun. "Kind" means a group held together by kinship, and it may be a larger or a smaller group held together by a close kinship or by a more distant one. "Sort," again, will not serve our purpose as an English translation of "species." For, although "a sort" implies a certain selection and similarity of the things included in the "sort," the amount of similarity implied may be very great or it may be indefinitely vague and remote. Hence naturalists have to stick to the word "species," and to use it with a clear definition of what they mean by it.

Suppose we get together a large unsorted collection—many hundred "specimens" or individuals—of the common butterflies of England. Then, if we look them over, we shall find that we can pick out and arrange the specimens into definite groups, according to their colour-pattern. We find that the kinds which we readily distinguish are called in English the swallow-tails, the whites, the sulphurs, the clouded yellows, the tortoise-shells, the peacocks, the red admirals, the painted ladies, the gatekeepers, the meadow browns, the heaths, the coppers, and the blues. There might be others in such a collection, but that is enough for our purpose. On examining the specimens closely, we find that the colour-markings and "venation" or network by which the wings are marked and the shape of the wings, body, and legs of all the specimens of the swallow-tails are almost exactly alike, and unlike those of any of the others. We shall find if we have a dozen or two specimens that there is a slight difference in the pattern, size, and colour of wing of some of the swallow-tails, dividing them into two groups, which we soon ascertain to be the males and females; but this is so small a difference that we may ignore it. The swallow-tail is obviously and at once distinguished from any of the other butterflies in the collection by its colour-pattern and shape. So also with the others, there will be many specimens in each case agreeing in colour and pattern, and recognizable and distinguishable from the rest by the colour-pattern and by the "venation" or "nervures" of the wings. If we collect butterflies again in other years and in other parts of the country, we find the same set of shapes and patterns exactly, corresponding to what we have learnt to call swallow-tails, whites, sulphurs, clouded yellows, tortoise-shells, etc. There are, we thus learn, several distinct, unchanging kinds of butterfly, which are common in this country, and appear every year. Similarly we may go into a meadow in spring, and gather a number of flowers, and a naturalist will roughly arrange our bouquet into "kinds"; there will be the buttercups, the daisies, the clovers, the dead nettles, the poppies, the roses, the orchids, etc.

If, now, we look more carefully at our collection of butterflies, sorted out roughly into kinds or species, we shall find that the "whites," although holding together by a close similarity in having merely white wings edged and spotted with black, yet differ amongst themselves, so that we distinguish a larger kind, the large garden-white, and a smaller, commoner kind, the smaller garden-white, and we distinguish also the green-veined white, and possibly the rare Bath white, each of them differing a little in their spots as well as their size. These different sorts of "whites" can, once our attention is drawn to the matter, be readily distinguished from one another, and constantly are found in our collections. We thus arrive at the conclusion that, though the whites are much alike, and are a kind distinct from the other kinds of butterflies, yet the "whites" themselves can be divided into and arranged as several kinds distinct from one another. In fact, we discover (and an illustrated book on butterflies confirms us in the conclusion) that there are several ultimate kinds of whites which cannot be further separated into groups. These are what are called "species." The whites are therefore not a single species, as are our British swallow-tails, but a group of species, closely related to one another. We find the same thing to be true with regard to the blues. Though they are much alike, agreeing in a variety of details of spotting and colour, yet we can distinguish the chalk-hill blue, the common blue, the azure-blue, the Adonis blue, and others, as distinct "species" of blues. Then, again, when we carefully examine our English specimens of tortoise-shells, we find that there are two distinct "species"—the greater and the smaller—differing not only in size, but in pattern; and when we compare with these the painted lady and the peacock and the red admiral, we find that there is a certain agreement of wing-pattern (venation and outline) and details of shape among them all, although their tints and the shape of the spots and bands of colour differ. These different species "hold together" just as the whites do and just as the blues do. Naturalists have met the need for expressing this similarity of a number of distinct species to one another by introducing the term "genus" for such a group. In fact we arrange several species into a "genus." The "genus" is a "kind," but a more comprehensive "kind," than is a species. The species is an assemblage of individuals closely alike to one another; the genus is a group of species which are more like to one another than any of them are to other species.

Naturalists give to every genus a name, and also a name to each species in the genus. Since we naturalists want to know what butterflies or other species of animals and plants are found in other countries, and to be sure that we all (whatever our native language may be) mean the same thing by a name, Latin names are given to the genera and the species, and are necessarily used when one wishes to be sure that one is understood. The greatest trouble is taken to make certain that the name used is applied only to the original species and the original genus to which it was applied, for only so can one be sure that a writer in America or one in Italy or France means the same thing by a name as we do here in England. This is rendered possible and is actually brought about by the preparation of catalogues in which the species are described and figured, especially with regard to obvious points of detail which are constant, and are called "specific characters." These are chosen for special description, not haphazard, but with a view to their being recognized with certainty by those who study other specimens. Another extremely important proceeding in connection with this purpose of uniform naming, which involves vast labour and expense, is the maintenance of great collections of preserved animals and plants by the State in all civilized countries. In these collections either the original specimens to which names were given by recognized describers (called "type-specimens" or "the type") are preserved, or else specimens which have been compared with those original described specimens, and authoritatively ascertained to be the same as the "type." The maintenance of accuracy and agreement in regard to the names of all the "species" of plants and animals is a big task. It is now carried out by international councils, in which the skilled naturalists of the world are represented. Certain principles have been agreed upon as to the method of determining the priority of one name over others which have been employed for one and the same species by naturalists of different countries and at different times, and a general agreement as to what names are to be used has been arrived at. It is a matter which has involved a great deal of uncertainty and dispute, and still causes difficulty. By the exercise of good sense, and in consequence of the existence of an urgent desire really to understand one another, there is now every year an increasing uniformity and agreement among naturalists about the exact name to be applied to every species of living thing.

Returning to our collections of butterflies and meadow flowers, we may take the names of some of the species and genera as an example of the system of naming in use by scientific naturalists. The common swallow-tail is assigned to the genus Papilio. Its "specific name" is "Machaon," given to it by LinnÆus, hence it is spoken of as Papilio Machaon. It is found in various parts of Europe as well as in England. But in Central Europe (often seen in Switzerland) there is also another species of swallow-tail, which only occurs as a rare accident in England. This is the pale swallow-tail, differing, not only by its paler colour but by definite spots and markings of the wings, from the English species. Its species name, or "specific name," is "Podalirius," and so it is known as Papilio Podalirius. Species of Papilio are found all over the world; more than 500 are known. Our two commonest whites belong to the genus Pieris—they are called respectively Pieris brassicÆ (the larger) and Pieris rapÆ (the smaller). The green-veined white is Pieris napi. Each of these three is called after the plant, cabbage, rape, or turnip, on which its caterpillar feeds. The rare Bath white is Pieris daplidice. Its caterpillar feeds on mignonette. There are dozens of species in other parts of the world allied to our "whites," which naturalists have carefully distinguished and characterized by their marks.

Several of our most beautiful species of English butterflies which are much alike have been enrolled in one genus—the genus Vanessa. This genus includes the great tortoise-shell, called Vanessa polychloros; the smaller tortoise-shell, Vanessa urticÆ; the peacock, Vanessa Io; the painted lady, Vanessa cardui; the red admiral, Vanessa Atalanta; and the comma butterfly, Vanessa C-album. There are other European, Asiatic, and American species of Vanessa.

In the same way we find with our meadow plants that what we at first thought was a single kind, "the" buttercup really bears a name applicable to a genus in which are several common species. The genus is called Ranunculus, and there are several common English species with yellow flowers, but distinguished from one another by definite characters. They are Ranunculus acris, Ranunculus flammula, Ranunculus bulbosus, Ranunculus arvensis, Ranunculus ficaria (the lesser celandine). And then there is the white-flowered Ranunculus aquatilis—a common pond plant. Clover, again, is by no means the name for a single species. The clovers form the genus Trifolium, and in any English meadow we may come across the white clover, Trifolium repens; the red clover, Trifolium pratense; the hop clover, Trifolium agrarium: the strawberry clover, Trifolium fragiferum; the haresfoot clover, Trifolium arvense. So it is with the plants which at first sight we distinguish merely as "daisies." There are several distinct genera of daisies—Aster, Bellis, Chrysanthemum (ox-eye), Anthemis (camomile), and others, with several distinct species in each genus.

Enough has been said to show the reader that the mere notion of "kinds" does not carry the same meaning as "species," but that there are a number of regularly occurring definite forms of both animals and plants which can be arranged in groups consisting only of individuals which are very nearly identical with one another. A group of living things of this degree of likeness is called "a species," and receives a name. A less degree of likeness holds together a number of species to form what we call a genus, and the name of the genus is cited together with the name of the species when we wish to speak of the species with clearness and certainty. This system of double names we owe to the great Swedish naturalist of the eighteenth century, LinnÆus. He proposed also that the relationships of living things to one another should be further expressed by grouping like genera into "families," then like families into "orders," and like orders into "classes." And since his day we go further and group classes into "phyla" or great stems of the animal pedigree. In this way a complete hierarchy or system of less and more comprehensive groups has been established, and is the means by which we indicate the natural groups of the family-trees of plants and of animals, what, in fact, is called the "classification" of each of these great series of living things. LinnÆus compared his system of groups to the subdivisions of two armies. Thus, the one army represents the whole animal series, the other the whole vegetable series. An army is divided into (1) "legions," these into (2) "divisions," "divisions" into (3) "regiments," regiments into (4) battalions, and battalions consist of (5) companies, consisting of individual soldiers. According to LinnÆus, we may compare the legions to classes, which are divided into orders, comparable to divisions; these into families, comparable to regiments; these into genera, comparable to battalions; and these into species, comparable to companies, or ultimate groups of individual units or soldiers.

Just as the legions, divisions, regiments, battalions and companies of an army have each their own name or at any rate a distinctive numeral assigned to them in order that they may be cited and directed, so are names given to each class, order, family, genus and species of the classification or enumeration of the kinds of animals and plants. Here, for instance, are the names of the greater and smaller groups in which our common "white" finds itself enrolled. Class—Insects. Order—Lepidoptera. Family—PieridÆ. Genus—Pieris. Species—brassicÆ.