MORE ABOUT SPECIES

I WROTE in the last chapter of the recognition of that degree of "likeness" or kinship in animals and plants which we point to by the word "species," and of the grouping of several similar species to form a "genus," and of several genera to form a family, of families to form orders, and of orders to form classes—and of the giving of names to all these groups. Whilst the making of this or that lot of species into a distinct genus, and giving it a new name is a mere matter of convenience for the indication of more or less important agreements and divergences, and is to a large extent arbitrary or an expression of opinion—it has always been recognized among naturalists that the group called "a species" is not a mere convention, but has a real natural limitation. It is true that the actual things which we see in studying natural history are so many units or individuals. But the possibility of arranging these by pattern, colour and shape into ultimate companies of which all the units are alike and differ from all the units of another company, has been regarded as a natural fact of primary importance and not a mere convention or convenience. The conception of the "naturalness" of a species depends really upon a further qualification of great importance as to what we naturalists understand by it.

We find by rearing plants from seed and by causing animals to breed under actual observation that the individuals of a species pair with one another, and not with individuals of other species, and further, that the young which they produce are like the parents—show themselves, in fact, to be of the same "species." The species continually year after year reproduces itself with little variation, though some variation does occur. The faculty of pairing only within the group, of never naturally breeding with members of other groups, has accordingly been adopted as a test of species. Hybrids between two species do not occur, except in very rare cases, in the state of nature. It is not always the case that the members of two species cannot possibly pair together, but it is the fact that they do not do so. Man sometimes brings about such crossing or hybridization, and it is a curious fact that the hybrids are often infertile or give rise only to weakly offspring, which could not survive in the natural struggle for existence. Sometimes, however, when the two hybridized species happen to come from regions of the world remote from one another, the resulting hybrids establish a vigorous race. There are real obstacles (of which I will say more below) in natural conditions to hybrid-breeding between any two species which occur naturally in the same territory. Thus the idea of a species is expanded so as to be not merely "a group of individuals of constant likeness in form and characteristics," but we add to that definition a living or constitutional quality expressed by the words, "which produce fertile offspring by pairing with one another, but do not pair with the members of other species."

This enables us to distinguish the conception of a "species" from that of a "variety" or a "race." We find occasionally peculiarly-marked examples of a species of plant or animal, or even local races of peculiar form; but we do not regard them as "distinct species" if we find that they breed as a rule with the ordinary members of the species. The decisive test is the breeding. If the variety is found not to breed with the regular species, but to keep apart and breed only with other individuals like itself, then we say, "This is no mere variety! It is a distinct species!" Unfortunately we have vast series of animals, insects, and others, from all parts of the world, collected and preserved in our museums, of which we know only the dead preserved specimens. So that we cannot be sure in doubtful cases whether a series of forms differing a little from the ordinary members of a species indicate distinct species, as defined and tested by breeding. We have in such a case to note the difference, and record it either as a variety or as a species by a guess at the probabilities one way or the other. Naturalists really intend by the word "species" to designate a form represented by numerous like individuals, which, in the present natural conditions of the region they inhabit, have attained a certain "stability" of distinctive form and character (not without some variability within definite limits) and constitute a more or less widely distributed population, the members of which inter-breed but do not produce offspring with other allied species.

A good case by which to exhibit further our conception of a species is that afforded by the species which are united in the genus Equus—the horse-genus. There are living at the present day several wild kinds of Equus—namely, the wild horse, or Tarpan, of the Gobi desert of Mongolia, called after the Russian explorer Przewalski; two kinds of Asiatic wild ass, called the Kiang and the Onegar; the African wild ass, and two or three kinds of zebra. There are, besides, many kinds of domesticated horses, ranging from the Shetland pony to the Flemish dray horse, and from the Shire horse to the Arab. Then there are many kinds of fossil extinct horses known, some of which clearly must be placed in the genus Equus with the living kinds; others which have to be separated into special genera (Hippidium, Onohippidium, etc.). Now, as to the living forms or form-kinds of the genus Equus—which are we to regard as true species, and which are only varieties and races of lower significance than species? The answer is clear enough in regard to several of them. The wild Mongolian horse and all the domesticated horses are varieties, races, or breeds of one species, judged not only by such marks as the possession of callosities on both the hind and the fore legs, but also by the test of breeding. They breed together and produce persisting races. But the asses and the zebras, though they will form mules with the horse, do not in a state of nature freely breed with it. When an ass or zebra is mated by man with the horse it will produce hybrids, called "Mules," but will not in "a state of nature" establish a hybrid race. The asses and the zebras are distinct from the horse, not only in markings and certain details of shape and hair, but in the fact that they cannot be fused into one race with him. There are no sufficient experiments on the aloofness of zebras and asses from one another in regard to breeding, although it seems that they cannot establish a mixed race, and are, therefore, distinct species judged by that test as well as by their form and marking. It is not known whether the so-called species of wild ass—the Asiatic and the African—would prove to produce fertile or infertile mules if intercrossed, nor has the test been applied to the very differently-marked local races of the African zebras—Grevy's zebra, Burchell's zebra, and the mountain zebra. It is likely enough that the three or more "species" distinguished among zebras on account of their being differently striped, and existing in different localities, would be found to breed freely together, and prove themselves thus to be entitled to be regarded as local "varieties" or "races," but not as fully-separated true species.

Thus one sees how difficult it is to have knowledge of the breeding test, even in regard to large animals. It is obvious that the difficulty of obtaining it in regard to the thousands of kinds of minute creatures is much greater. Yet when they say, "This is a distinct species," naturalists do mean that it is not only marked off from other animals or plants most like to it by a certain shape, colour, or other quality or qualities, but that it breeds apart with its own kind and does not naturally hybridize with those other forms most like to it.

Although the kind of naturalist called a "systematist" who makes it his business to accurately describe and record and distinguish from one another all the existing species of some one group—say, of antelopes, of mice, of flowering plants, of fishes, or of fleas—has only a knowledge in a few instances of the breeding of the organisms which he describes as "distinct species," he yet does know, in regard to some one or more of his species in most groups, the facts of pairing and reproduction, and what are the limits of variation in the markings and other characteristics of at least one or two species definitely submitted to the "breeding test," that is to say, ascertained to be "true physiological species," kept apart by deep-seated chemical differences in their blood and tissues. Hence it is legitimate for him, by careful balancing and consideration of all the facts, to determine—not absolutely, but by analogy—the value to be assigned (whether as indicating true species or merely varieties capable of pairing with the main stock) to points of difference among the specimens of a dead collection brought from some distant land or from some position in which it would be impossible to make observations with regard to "pairing" and "breeding true."

Some 400 species of fleas have been described, and we are certain as to the value of the characters relied on to distinguish those species, owing to what we know of the breeding of some common species of fleas. The flea of the domestic fowl, that of the domestic pigeon, that from the house-martin, and that from the sand-martin—used to be considered as one species until they were carefully examined twenty years ago. In reality each of them has its own peculiar "marks," and they do not mix with one another. The nests of the sand-martin yield only one species of flea, namely that peculiar to the sand-martin. The hen-house, the dove-cote, and the nests of the house-martin yield each their flea maggots, which can be reared and become in each case a distinct species with definite recognizable "characters." On the other hand, the flea of the rabbit gives an opportunity of studying the limits of variation in a "good" species. Rabbit warrens swarm with the rabbit flea, and often a great number are found on one rabbit, the individual fleas "varying"—"differing" from one another to a slight extent. The "systematist" thus gets to know what organs are variable within the limits of an undoubted physiological species of flea, and what are comparatively constant—so that he can form a reasonable opinion about the claim of other specimens which he may receive without full history of their habits, to be regarded as true distinct species.

The fact that most important chemical differences of the blood and digestive juices often accompany the small external differences which enable us to distinguish one species of animal or plant from another, makes it obvious that the knowledge of species is a very valuable and necessary thing. One species of flea, the Pulex Cheopis, habitually carries the plague bacillus from animals to man, and is a cause of death; other species, extremely like it in appearance, but distinguishable by a trained observer, do not carry the plague bacillus, but if they swallow it, destroy it by digestion. One species of gnat, the common grey gnat, digests and destroys malaria germs when it sucks them up with blood; in an allied species, the spot-winged gnat or Anopheles, the chemical juices of the gut allow the germ to live in it and multiply, and so to be carried to men by the gnat's bite. So with many other flies and parasites the recognition of the dangerous species is of vital importance, and that recognition often depends on minute features of form and colour not at once obvious to an ordinary observer.

But this recognition of distinct species is, from the point of view of the study of Nature, only a preliminary to the question, "How did these species come about? How is it that there are so many species, some very like one another, forming genera, and these genera grouped into related families, these into larger groups, and so on, like the branches of a family tree?" The answer to these questions given by LinnÆus was: "There are just so many species as the Infinite Being created at the beginning of things, and they have continued to propagate themselves unchanged ever since." The answer which we give to-day is that the appearance of a huge family tree which our classification of animals takes is due to the simple fact that it really is neither more nor less than a family tree or pedigree—the "tree of life," of which the green leaves and buds are the existing species. Further, we hold that the existing species of a genus have "come into existence" by natural birth from one ancestral species, its offspring having slightly varied (we are all familiar with this individual variation in our own species, in dogs, cats, trees, and shrubs), and that the varieties have wandered apart and become continuously emphasized and selected for survival by their fitness or suitability to the changed conditions around each of them. Meanwhile a natural destruction, or failure of intermediate forms to survive, has gone on.