III.

THE CONSTITUTION OF THE ANIMAL AND

PLANT KINGDOMS.

THE intelligent foreigner, visiting a country which to him is practically a terra incognita, and desirous of acquainting himself as fully as possible with the constitution of the land wherein he intends to sojourn, would contrive, before departing from his native coasts, to gain some adequate idea of the new country itself, its government and laws, its social, political, and religious condition, its geographical and geological features, and its general history in so far as these details were necessary for the comprehension of what he expected to see and hear during his foreign tour. If to the details of its present condition he was able to add information concerning its past—if he could trace its history along the lines of centuries, and discover how this event or that occurrence had tended to mould the country and its constitution into its existing form, his appreciation of the strange land, as presented to his view to-day, would tend to become of still more complete nature. And if, lastly, from his study of the past and present of the foreign territory, he ventured to indulge in any reflections on its possible or probable future, and on its chances of further development or possible decline, such reflections would possess every claim to rank as rational thoughts, deducible from his knowledge of the land as it was and is.

The parallelism between the process of acquiring an adequate knowledge of a foreign state, and that of gaining some idea of the constitution of the worlds of living beings, can readily be shown to be of the closest possible description. The most superficial acquaintance with the study of zoology and botany, if carried out in any fashion worthy the name of a scientific and intellectual exercise, must proceed along lines which follow out in all essential details the pathways whereby we gain an intelligent idea of a foreign land. No study of animals or of plants can be satisfactorily carried out without, at least, a brief preliminary discussion of the constitution of the worlds of life, and without some acquaintance with their mutual relationships and their fundamental characters. In the light of recent researches concerning the "why and wherefore" of the animal and plant kingdoms, such preliminary knowledge becomes not merely of high importance, but of absolutely essential

nature.

In the days before "evolution" was anything but a name, and ere "Natural Selection" had become a striking reality to the biological mind, such knowledge formed the basis of every study of zoology and botany worthy the name of a scientific investigation. To-day, when the burning questions of biology centre around the evolution of the living universe, and include in their sway and limits the details of the development and structure alike of man and monad, it need hardly be urged that some acquaintance with the general constitution of the animal and plant worlds is absolutely necessary for the intelligent comprehension of all that is interesting in the study of life. If the "making of England," to quote the expressive phraseology of a historical authority, be regarded as at once the summation and foundation of all knowledge of the genesis of the English race, so the fundamental nature of animals and plants and a knowledge of their existing relations may be legitimately viewed as the only sound preparation for a knowledge of the great questions that deal with the becoming and making of living things.

The most cursory survey of the worlds of animal and plant life leaves, as the prevailing impression on the observer's mind, the idea of extraordinary variety and diversity of form, colour, and habitat. From the grand Sequoia (or Wellingtonia) of California to the humble moss that covers a rock, the grey lichens of the walls, or the minute Alga that colour the pools, there is an endless variety in the ranks of the plant kingdom. No less distinctly is the diversity seen in the hordes of animal life. From the giant quadrupeds that find a home in the tropical jungles, through the teeming life of the waters, to the insect life that everywhere surrounds us, and to the animalcular swarms that find a world in the water-drop, there is to be viewed endless and well-nigh undeterminable variation in every feature of existence. Indeed, so wide is the range of the naturalist's sphere of observation, that one might be readily tempted to believe that, save for the one common belonging and possession of life, there seems no bond of union which may link together the hosts that people the earth. The variety in question tends somewhat to puzzle the uninitiated observer when he attempts to form some adequate ideas regarding the relations of animals to each other, or concerning the bonds that connect the apparently diverse forms of plant life. It is this variety also, which in some degree tends to discourage the popular study of natural history-the apparent hopelessness of overtaking in a human lifetime even a small portion of the inexhaustible fields of research, having its own share in the work of discouragement, and in demonstrating the theoretical vanity of human knowledge. But the student of living nature is destined to find a speedy and satisfactory solution of many of these preliminary difficulties at the very outset of his studies. The first tendency of

scientific investigation is to correlate the objects of its research; or in other words, to effect a classification and arrangement of subjects destined for investigation. When the child groups the objects by which he is surrounded into animals, vegetables, and minerals, he is unconsciously laying the foundations of a scientific system; and, in reality, the naturalist simply enlarges the conceptions of the child when he shows that differences, as fundamental in their nature as those the child learns to note, can be determined between the varied tribes of animals and the equally diverse groups of plants.

Prior to the time of Cuvier, naturalists concerned themselves chiefly with the description of the different species of animals and plants, and with the determination of the characters whereby one species was distinguished from another. The writings of Linnæus, for example, are largely composed of such descriptions, and if we add to such details, others dealing with the habits and distribution of animals and plants, we shall have completed the enumeration of the chief aims of naturalists in bygone days. The popular zoology and botany of to-day, which do not concern themselves with matters beyond form and the recognition of species or the description of habits, reflect, in a very characteristic and exact fashion, the natural history studies of the past. It should be remembered, however, that the classic naturalists, amongst whom Aristotle stands out conspicuously, dived somewhat more deeply into the history of the animal kingdom than their modern successors. But it may be fairly assumed that the ordinary naturalist, prior to Cuvier's time, concerned himself not so much with the structure or morphology of living beings as with the description of their external forms, peculiarities, habits, and habitations.

With Cuvier, a new and higher era of natural history study dawned. Linnæus had mapped out the animal world into (1) Mammalia, (2) Aves, or birds, (3) Amphibia (reptiles, frogs, &c.), (4) Pisces, or fishes, (5) Insecta (insects, spiders, &c.), and (6) Vermes, or worms-this latter group being, like that of the Linnæan "Insects," a most heterogeneous division, and including all known and lower forms of animal life, from the " worms themselves downwards as far literally as the senses could reach. It has well been remarked that such a classification as the foregoing possesses a representative in the vocabulary of well-nigh every language. In this view it might be maintained that a popular conception of a unity of animals underlying their obvious diversity was early formed in the human mind. This is undoubtedly true, since the division of the animal world into beasts, birds, fishes, insects, and worms is a step in the construction of animal "types," to one or other of which any animal may be referred. But the system in question exhibits, after all, but little advance on the classification of

childhood; and it serves, moreover, to indicate very cursorily indeed the scientific and further delineation of the animal constitution. Lamarck, whose name is associated with views concerning the transformation and evolution of species, contributed a very decided addition to the knowledge of the constitution of the animal world, when, about the close of last century, he showed that the beasts, birds, reptiles, and fishes, instead of being regarded as distinct and unconnected divisions of animals, might be grouped together to form a large and characteristic division of the kingdom. pointed out that each and all of these animals, as he knew them, possessed, firstly, a spine or backbone. Within this spine (Fig. 1, A p), whereof the skull formed merely a front expansion, the nervous system (n2) was con

He 112...

p

ho

A

-C

B

tained as within a tube; whilst FIG. 1.-CROss-Section of Vertebrate (a) below that system, and contained

AND INVERTEBRATE (B).

within the body (2) itself (as bounded, say, by the ribs), were the heart (h), digestive system (a), and other organs. Lamarck, commenting upon this arrangement of parts-which a glance at the carcase of a sheep, as vertically bisected in the butcher's shop, will illustrate-demonstrated that no other animals, save mammals, birds, reptiles, and fishes, possessed such a disposition of their organs. The worm or the insect, for instance, possesses a body (Fig. 1, B) we may legitimately compare with the lower tube (A) of the fish or beast, since neither the worm nor insect has a spine containing a nervous system. Hence Lamarck, taking his chief character from the spine or backbone, composed of separate bones or vertebræ, named the beasts, birds, reptiles, and fishes the Vertebrata, whilst all other animals became accordingly known as Invertebrata.

That Lamarck's discovery and his subsequent arrangement of the animal world into these two leading divisions marked a distinct era in zoology no one can doubt. Best of all, his deduction laid the foundation of the method which a little later—that is, about 1795Cuvier so successfully enunciated and followed out to a practical result. Other hands, in addition, laboured at the scientific edifice, which was practically completed when Cuvier laid before the world his elementary scheme of the history of animals, and showed that at least three common types or plans could be instituted amongst the invertebrate animals. Placed in tabular order, then, the main outlines of the animal world, according to Cuvier, might be thus rendered :I. VERTEBRATA ("backboned" animals) (fishes, frogs, reptiles, birds, and mammals).

II. MOLLUSCA ("soft-bodied" animals) (cuttle-fishes and "shellfish" at large).

III. ARTICULATA ("jointed" animals) (insects, crustaceans, worms, &c.).

IV. RADIATA ("rayed" animals) (star-fishes, corals, jelly-fishes, zoophytes, and all lower animals).

Cuvier's own words, expressive of the nature of these types, may be quoted: "It will be found that there exist four principal forms, four general plans, if it may thus be expressed, on which all animals appear to have been modelled; and the ulterior divisions of which, under whatever title naturalists may have designated them, are merely slight modifications, founded on the development or addition of certain parts." It may be added that the distinguished embryologist Von Baer, attacking the problems of animal form from the standpoint of development, and watching the phases observable in the early history of animals as they advanced from the egg towards their perfect forms, came to the same conclusion as the great French anatomist. According to Von Baer, also, there were four types or plans in the animal world, the distinctive nature of the type to which any given animal belonged being indicated at an early stage in its development. So that, as early as the beginning of the present century, it became clear to the minds of naturalists that, instead of each animal being built up on a type peculiar to itself, it fell into one or other of four groups; in a word, it was found to possess a broad and fundamental plan or type of structure, with which a greater or less number of other animals agreed.

To render the "type" constitution of the animal world plainer and more readily appreciated, we may select one or two examples by way of illustrating, also, how, with the increase of knowledge since Cuvier's days, the original types have remained stable in some respects, whilst they have undergone modifications in others. No two animals can well appear more varied in form, nature, appearance, and habits—and inferentially in structure likewise-than a lobster and a butterfly. The aërial habits of the one contrast very markedly with the slow aquatic life of the other, whilst the general constitution of the former appears to be separated by antipodean differences from that of the other. Are there any bonds of common nature which can link together beings so diverse? and can the butterfly and the lobster be shown to possess any relationships in common? are questions which it is reserved for the scientific but plainly understood deductions of zoology to answer. The superficial examination of the lobster would show that its body consists essentially of a series of some twenty joints, each possessing a pair of appendages modelled, despite their apparent differences, on one and the same