This brief sketch of the four great questions of biology may serve to show the exact position which the study of Distribution bears to the other departments of natural-history research. Taking its stand as a distinct branch of inquiry; dealing with the causes which have placed animals and plants in their distinct regions; investigating the conditions which make for or contend against the diffusion of animals and plants on the surface of the globe-the science of distribution presents problems and attempts the solution of questions involving, it may be, the furthest knowledge of present and past alike, which is at our command. Nor must we neglect to note that the study of distribution relates that present history, in the most intimate fashion, with the past of the globe. The continuity of the past with the present is too much a ruling idea of the biological mind to allow the importance of the geological factors in the world's problems to be overlooked. Not a few of the knotty points of distribution are soluble from the side of geology alone. If, therefore, for no other reason than that it links present and past so intimately together, thus making the unbroken continuity of causation a necessity in biological explanation, the study of distribution would take its place in the first rank of the sciences of to-day Bearing in mind this twofold division of distribution into that in space (or "geographical distribution") and that in time (or "geological distribution "), we may now profitably proceed to inquire into the history of the growth and progress of this department of inquiry.

If we turn to text-books on natural history, written even some ten years ago, we shall discover that, whatever may be the importance of the study, the science of distribution is of comparatively recent growth. The information dispensed in these manuals of biology resolves itself for the most part into a brief recital of the countries in which different animals and plants are found. Thus the facts of distribution, which an intelligent child is now taught in the nursery, comprehend all that was known, even in recent science, respecting the habitats of animals and plants. To know that lions occur in Africa, and tigers in India; to learn that the giraffe and the hippopotamus are tenants of Ethiopia, and that rhinoceroses occur both in Asia and Africa; to be able to say definitely that kangaroos never occur without the bounds of Australian islands, or that humming-birds are found in the New World alone; to know where palms grow or where cacti abound-these were the only facts which the "distribution" of twenty years ago included. The plain enumeration of these or any other facts, however, does not raise them to the rank of a science. The mere mention of the detached countries in which

plants and animals occur, does not constitute a philosophical piece of information calculated to explain either itself or any correlated facts of natural history. That method alone converts any body of details into a science, which places them in harmony with each other, and which, connecting them by, it may be, even a transcendental bond, links them together as parts of a whole. To know, for example, that the existing horse walks upon the greatly developed third toe of each foot, to become aware that the horse likewise possesses two rudimentary toes on each foot, are mere facts, valuable enough perhaps in themselves, but useless, so long as they remain isolated, for any higher or philosophical reasoning concerning the horse or any other animal form. Once, however, let these facts be placed in true harmony with other details regarding the equine race, and the science—that is, the true knowledge of horses is then constituted. Thus, if we discover that the horses of the present are connected by a complete series of gradations with the horses of the past; and that we may pass by graduated stages from the onetoed horse of to-day to the five-toed Mesozoic ancestors of the race, we at once rise into the region of a philosophy which, through correlated facts, seeks to teach us the origin of the equine species. If, further, knowing that horses were believed to have first been introduced into the New World at the Mexican Conquest, we suppose that in its distribution the horse is a strictly Old World form, that isolated fact tells us but little of the history of the race. Even if we discover that the fossil remains of horses occur in the Tertiary deposits of America as well as in those of Europe, the knowledge of that fact may certainly enlarge our ideas of the form.er distribution of horses, but of itself the fact does not place us in possession of any connected details concerning the general history of the form in question. But when, by bringing these varied facts into relation with each other, we seek to construct a pedigree of the equine race, we then illustrate the higher use of our knowledge, in that we cause that knowledge to explain itself.

Of all the facts of distribution, the same opinion may be expressed. Formerly, to say that a given animal was found in this land or that, was accounted the beginning and end of distributional science. The influence of evolution, and the growth of newer ideas concerning the modification of species, have together created for us a literally new science of distribution. The ideas which prevailed a quarter of a century ago regarding the fixity of species, and the consequent fixation within certain limits of their habitats, demanded no further exercise of scientific acumen than that necessary to say VOL. CCLIII. NO. 1819.

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from what region any given organism was derived, or from what tracts it was absent. With altered ideas of the constitution of the animal and plant worlds, higher and better because truer conceptions of the manner and causes of the distribution of life on the globe grew apace. In the days of Edward Forbes, the doctrine of "specific centres" held its own as representing the foremost science of its day and generation. With the dogma of the special and independent creation of each species of living beings left utterly unquestioned, it was of all logical processes the most natural that a "special centre" of creation should be sought and found for each species. This theoretical "specific centre" was allocated, cæteris paribus, in the region where the species was found to be most abundantly represented. The diffusion of a species beyond its centre was due, it was held, to such favouring influences as continuous land surfaces, the presence of food in surrounding regions, The favourable temperatures and climates, and like conditions. limitation of a species to its centre or original arca was held, conversely, to depend upon an absence of the conditions favouring migration and dispersion. The presence of rivers, lakes, or seas, the existence of land-barriers in the shape of mountain-chains, extremes of temperature and vicissitudes of climate and other causes, were regarded as the means whereby a species was confined more or less strictly within its area.

But the growth of the idea that the existing species of animals and plants were the descendants, by ordinary generation, of preexisting species, wrought a wonderful and sweeping change in biological opinions concerning distribution, as in every other department of natural history science. The theory of the separate and detached placing of animals and plants here and there over the surface of the earth, in obedience to no ascertainable law, was soon driven to the wall as a weak invention possessing no logical standpoint whatever. Affording no reason for the marvellous diversities of life's distribution, the doctrine of "specific centres" was soon consigned to the limbo reserved for the myths and traditions of biology. To say that providential reasons—namely, the necessity of a fatty dietary on the part of the Esquimaux-accounted for the presence of seals and whales in the Arctic regions, or similarly, that farinaceous plants grew most plentifully in the tropics because the inhabitants thereof fed upon their products, might indeed satisfy primitive minds, preferring to bring scientific facts under the sway of dogma rather than to test dogma by the logic of facts. Moreover, all such apologetic attempts at correlating the facts of distribution with theoretical interpretations

of the designs of Providence missed their mark, because in placing man in the first place, and the distribution of life in the second, they reversed not merely the chronological order of affairs, but subverted the real aspect of the case. Thus, clearly, no explanation of the "whys" of distribution was forthcoming from former aspects of this study, just as the "hows" of the science were equally neglected. The newer era of research inaugurated by the publication and growth of Mr. Darwin's opinions, derived no small share of its power and progress from its ability to explain the "how" and "why," not merely of distribution, but of other departments of biology. Evolution, for example, gave a reasonable explanation of the metamorphosis or series of changes through which many animals pass, externally to the egg, in their development. The tadpole, as every schoolboy knows, grows to be a frog through successive changes converting it from a fish-like organism into the type of the air-breathing terrestrial adult. The caterpillar, through equally well marked alterations of form, becomes the butterfly or moth. Under the old idea of zoological causation, either form undergoes metamorphosis, because, to quote the words of Kirby and Spence, "it is the will of the Creator." "This, however," as Sir John Lubbock remarks, "is a confession of faith, not an explanation of metamorphosis." Evolution satisfactorily and finally replaces the want of rational ideas of metamorphosis by a higher idea of satisfactory causation, namely, heredity. The frog passes in its development through a metamorphosis, because its ancestor was a fish-like organism. It repeats, as an individual frog, the history of its race. So, also, an insect may directly or indirectly be credited with demonstrating, by the course of its development, its origin from lower stages of life. The development of every animal is a brief recapitulation of the descent of its species. Obscured, and often imperfect, that biography may be, but nevertheless it is plainly outlined before the seeking eye and understanding mind.

If evolution has thus assisted our comprehension of why an animal passes through apparently useless stages in the course of its development, no less clearly has that theory brought to light the meaning of the previously isolated facts of distribution. It was evolution which played to these facts the part of a guardian genius; marshalling their ranks into order and arrangement, and demonstrating that relationship between them which it is the province of science to explain. It is necessary to dwell upon the influence which evolution has exerted upon the study of distribution, simply because the latter science practically dates its origin from the day when the modifica

tion of existing species as a means of natural creation of new races of animals and plants was recognised. And it is with the greater satisfaction that one may dwell upon this mutual relationship of distribution and the theory of development, since the due appreciation of the clear explanation which the facts of distribution receive from evolution at large, constitutes a powerful counterproof of the truth of that theory. It is not surprising, therefore, to find Professor Huxley saying that "no truths brought to light by biological investigation were better calculated to inspire distrust of the dogmas intruded upon science in the name of theology, than those which relate to the distribution of animals and plants on the surface of the earth. Very skilful accommodation was needed," continues Huxley, "if the limitation of sloths to South America, and of the ornithorhynchus to Australia, was to be reconciled with the literal interpretation of the history of the deluge; and with the establishment of the existence of distinct provinces of distribution, any serious belief in the peopling of the world by migration from Mount Ararat came to an end. Under these circumstances, only one alternative was left for those who denied the occurrence of evolution-namely, the supposition that the characteristic animals. and plants of each great province were created as such within the limits in which we find them. And as the hypothesis of "specific centres" thus formulated was heterodox from the theological point of view, and unintelligible from the scientific aspect, it may be passed over without further notice as a phase of transition from the creational to the evolutional hypothesis. In fact," adds Huxley, "the strongest and most conclusive arguments in favour of evolution are those which are based upon the facts of geographical taken in conjunction with those of geological, distribution."

Or if we turn for a moment to the opinion of Mr. Darwin himself, we shall find an equally clear expression of the futility of the attempt to explain distribution on any other save an evolutionary understanding. In his classical work, the "Origin of Species," Darwin remarks the fact that "neither the similarity nor the dissimilarity of the inhabitants of various regions can be wholly accounted for by climatal and other physical conditions." He secondly notes the fact," that barriers of any kind, or obstacles to free migration, are related in a close and important manner to the differences between the productions of various regions;" and a third fact noted by Darwin is "the affinity of the productions of the same continent or of the same sea, though the species themselves are distinct at different points and stations." Again, Darwin remarks that, "in discussing