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inorganic things around. Hence we distinguish a third function of the living being ; that of innervation or relation. Exercised through the medium of a nervous system or its representative tissues, this function of relation regulates and controls, whilst it connects and harmonises, the other actions of which life's activities consist. The animal or plant, regarded from a physiological standpoint, lives thus a threefold existence, and performs a triple round of duties. It nourishes itself, it reproduces its race, and it develops and exhibits relations with its surroundings. The knowledge which demonstrates how these functions are performed answers the second of our four questions—“how does it live ?”
. Structure and functions, all-important as their detail may be for the understanding of animal and plant histories, do not, however, constitute or bound the entire range of biological observation. The inquiries of even the childish stage of man's culture concerning the living as well as the non-living universe, include, above all other points, the inquiry," where is it found?” Especially natural does such a question appear when applied to the living tenants of the globe. When we ask ourselves where any organism is found, in what quarter of the globe it is plentiful, where it is scarce, or where, lastly, it is never to be discovered, we are in reality approaching topics which lead us tolerably near to the ultimate questions of all biological study. It is the science of distribution which professes to answer the questions relating to the whereabouts of animals and plants in the world as it now exists, and in anterior epochs of our globe as well. Distribution thus includes two most natural divisions or lines of inquiry. It summarises the existing life of the globe in its inquiries regarding the geography of living things, or their distribution in space, as it is technically termed; whilst it no less succinctly attempts the solution of the problems relating to the past history of animals and plants, when, proceeding to avail itself of the information collected by geology, it pictures for us their distribution in time.
The knowledge of the structure, functions, and distribution of a living being, once comprehended all that science could hope to know of its history. Contenting itself with the fact that living beings are, biology might regard the knowledge which these three queries, “ what,” “how,” and “where” supplied, as all-sufficient for the furthest mental demands. But the newer epoch of biology includes a fourth question in its list of queries concerning living things. It presents for solution yet another problem, in the terms of which is focussed all the knowledge gained in other departments of biological research. This fourth query is that which demands to know how
the living being has come to be what it now is”-or “how it has attained to its present place and position in the animal or plant series.” The mere terms of such a question presuppose that the living population of our globe has undergone progressive development. It postulates change and alteration as natural conditions of existence, and it inquires how, in the case of each animal or plant, such change has operated-in what direction it has sped, and how it has affected and modified the living organism. Thus stated, there can be no difficulty in recognising the theory of evolution or development as that which purports to supply this mental demand, and to reply to the inquiry concerning the past history of animals and plants in relation to their present position and genealogical connections. Time was when the need for such a question was non-existent. So long as mankind regarded the world of life as presenting a fixity of constitution, there could exist no question of wide organic change for the biologist to meet and answer. With a firm and undisturbed belief in the special and independent “creation” of each species of living beings, the mind could experience no philosophic or other necessity for any inquiry into a past of modification and change. Possessing the idea that stability of organisation and form was the rule of existence, men had not learned to look for a past wherein, as in a glass darkly, might be discerned the birth of new species arising through the modification of the old. But the germ idea of such an evolution of life existed and prevailed long before the age which has seen its full fruition. Here and there evidence is to be found that, even in classic ages, the great problem of problems concerning the how and why of the universe itself was growing apace in the minds of men. Aristotle, remarking that rain falls not to make the corn grow, any more than it descends to spoil the crops, asks, “what therefore hinders the different parts (of the body) from having this merely accidental relation in nature?” So also Lucretius, in another department of inquiry, shadowed forth the atomic constitution of things, and paved the way for the thoughts of the after ages, when Lamarck, Erasmus Darwin, Goethe, and, in our own day, Charles Darwin, Wallace, and others, have busied themselves with the problems of the development of the teeming population of the globe. Thus arises the philosophic necessity for a fourth question—that of the ætiology or causation of living beings. This question, utilising all the knowledge gained by the sciences of structure, physiology, and distribution, endeavours to show how the organic world has grown and progressed towards the perfection it exhibits before our waiting eyes to-day.
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 forn.cr 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.
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, favourable temperatures and climates, and like conditions. The limitation of a species to its centre or original area 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 pre. existing 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