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close resemblance to the life of the Palæarctic region ; and which has, likewise, been to a slight extent modified by the migration northwards of southern forms. In the life of South America we perceive, on the other hand, the results of longer isolation and greater specialisation. There the development of special forms of life has accordingly progressed to a much greater extent than in North America ; and the effect of a commingling of types has been largely prevented by its relatively recent junction with Nearctic land. As in Australia, the lower types of quadruped life have been preserved by the isolation of that area, so in South America, the preservation of the sloths, armadillos, and anteaters, and the development of special forms of monkeys and other quadrupeds are to be regarded as the fruits of that separation which secures protection to lower and comparatively defenceless life.
A glance at some of the difficulties of distribution, and a reference to the influence of migration upon the distribution of life, may draw our consideration of this topic to a close. The progress of any science from the stage wherein it formulates its beliefs in theory, to that when its theories rise through cumulative proof into the higher region of fact, is not accomplished without trial and tribulation. Criticism, destructive and constructive, is the lot of every scientific theory. But the earnest and unbiassed mind welcomes the criticism wherein the trial of its beliefs is contained, as the honest mind gauges the tenability of its beliefs by the residue, large or small, of solid fact which it is able to collect after the critical assault upon its stronghold is overpast. Of the science of distribution it may be said that its evil days are fairly past. Critics it has had, and biological opinions may even now be found to differ regarding the minor details of its constitution. But the larger and more fundamental propositions of distributional science remain untouched. They have passed out of the sphere of discussion, and have taken their place amongst the stable facts of the scientific system. It is necessary, however, to detail one or two examples of the difficulties which may still disturb the complexion of the scientific mind, and which are ever welcome to the devotees of a science, since they afford the means whereby the weak points and the unsettled problems of the science may be strengthened and solved.
Of such difficulties, then, let us specify a few instances, by way of showing how readily their solution may, through careful consideration, be obtained. Mr. Sclater has specified in a highly distinct manner, a few of the knotty points that await the student of distribution, and has thus afforded opportunity for the discussion of the subject, and for their explanation or modification by the exercise of scientific acumen and research. Taking the case of the lemursthose curious quadrupeds usually classified as lower monkeys—we are presented with certain apparent anomalies in their distribution over the surface of the globe. Thus the lemurs have their headquarters in Madagascar, as already remarked, but they also occur in the Eastern Archipelago. They are scattered, to use the words of Mr. Wallace, “from Sierra Leone to Celebes, and from Natal to Eastern Bengal and South China.” How, it may be asked, can the apparently erratic nature of the distribution of these animals be accounted for? and how can the facts of such a straggling population be harmonised with those conceptions of orderly biological and physical laws on which the science of distribution bases its existence?
Mr. Sclater himself, in 1864, postulated the former existence of a continent occupying the site of the existing Indian Ocean. This continent, named Lemuria, he conceived might have formed the head-quarters of the lemur group, whence they became radiated and dispersed east and west. Such a hypothesis is no longer required, however, to account for the curious distribution of the lemurs. In the light of new facts, and especially in the face of geological evidence, the existence of the theoretical “Lemuria " is rendered unnecessary.
Mr. Sclater's perfectly justifiable supposition has simply been superseded by more natural explanations of the distribution of the lemurs, whilst the views entertained regarding the permanence of the great ocean basins and of the continental land areas are likewise opposed to the theory of a former land-connection between the Ethiopian and Oriental ter. ritories. For what are the geological facts concerning the range of the lemurs in the past? Their fossil remains occur in the Eocene rocks of France—that is, in the lowest and oldest of the Tertiary deposits the remains in question being those of a form allied to the existing "potto " of West Africa ; and in North America, where the lemurs exist to-day, the Lower Eocene rocks afford evidence of their existence in the past of the New World. So that we find at the outset our difficulties largely resolved by the bare mention of the idea that the existing anomalies in the range of the lemurs really depend upon their past history. In a word, as the “Marsupial” population of Australia is to be regarded as a survival, owing to land separation, of an animal class once world-wide in its range, so the lemurs now found at distant points in Africa and Asia, are merely the survivals of a lemurine family circle once represented both in the Old World and in the New. We know of their existence in Eocene times in Europe, and thence they probably spread in all directions—to Africa, Madagascar, Asia, and elsewhere. Killed off over the general area inhabited by their race, the lemurs have remained in the environs of the earth, so to speak, because there, to this day, the competition with higher forms is not too severe. Like the American opossums, the lemurs represent to-day the mere remnants of a once world-wide class. Their distribution has not been one from Asia to Africa, or vice versâ, through a once existent “Lemuria,” but has really been a diffusion from Europe, or the Palæarctic region probably, to the adjoining regions and to the New World.
A second case of difficulty in connection with the distribution of quadrupeds is that of the peculiar animals forming the order Insectivora, a group familiarly represented by the moles, shrews, and hedgehogs. This order of quadrupeds is highly singular in its range and distribution. It is entirely unrepresented in Australia and South America, and its representative species occur in the Palæarctic, Oriental, and Ethiopian regions; whilst North America also possesses moles and shrews, probably of very recent introduction into that continent. But more curious still is the fact that the Insectivora include certain peculiar and isolated animals, which inhabit detached regions, and which present problems for solution in the way of an explanation of the how and why of their existence on the earth's surface. For example, a curious animal (Solenodon) is found only in two West Indian Islands, namely, in Cuba and Hayti. Again, the nearest relations of Solenodon occur in Madagascar, where, under the name of the Centetida or “Madagascar hedgehogs," they flourish in numbers. Thus we are required to explain the following facts : Firstly, the detached existence of Solenodon in the Antilles; secondly, the similarly isolated distribution of the species of Centetes in Madagascar; and thirdly, the absence of any species of Centetes in the intervening African continent.
In attempting to solve these problems we find that the way of investigation lies along the same lines as those which lead to a solution of the case of the lemurs. The existing Insectivora are small animals, mostly living in areas where they are removed from the direct effects of competition with other and stronger forms. Their fossil history is fragmentary but important; for we discover a link that connects Solenodon of the New World with Centetes of the Old World, in the fossil Centetidæ which occur in European deposits of Lower Miocene age. With even this solitary fact at hand, we begin to discover that the problem before us is not the bridging of the gulf between the West Indies and Madagascar, but the simpler
task of accounting for the survival in out-of-the-way corners of the earth of a group once far more widely distributed. Thus Madagascar obtained its species of Centetes just as the West Indies obtained their Solenodon, namely, at a time when land-connection with a larger landarea permitted these insectivores to gain admittance to what was shortly to become a detached island-area. As has been pointed out, the conditions of life which exist in Madagascar closely resemble those of the Antilles, and both differ in turn from the conditions that prevail on the adjacent continents. There is an absence of large quadrupeds, a lack of carnivores, a complete separation from larger areas by deep sea, and, in fact, a full representation of all the conditions which suit these insectivores, just as conversely on the continents the conditions are unfavourable to the prosperity and increase of their race. We do not require to connect the Antilles and Madagascar on account of these animals, any more than we need to postulate the existence of a former Pacific land-connection between Asia and America because the camels of the former continent are related to the llamas of the latter. And when we further reflect that Madagascar preserves a mouse nearly related to a New World type, and snakes belonging to a typical American group, we at once note how the principle of seeking to prove the former wide distribution of a race of animals and its modern limitation by geological and biological changes forms the best clue to many of the difficulties of this science. It is a clue, moreover, which is at once originated and supported by the fossil histories of the animals whose distribution is the subject of remark.
A third case which has excited the attention of students of distribution is that concerning the past history of the giant tortoises found in the Mascarene and Galapagos Islands—the former belonging to the Madagascar group, and the latter being situated 600 miles from the South American coast. Of these tortoises, as Dr. Günther has shown us, three chief groups exist. One of these inhabits the Galapagos, a second occurs on the coral island of Aldabra to the north of Madagascar, and a third, which has become extinct, inhabited the Mascarene group of islands. But our difficulties are lessened in this case—which demands the explanation of the existence of apparently similar forms in widely-removed areas-by the knowledge that these tortoises, though apparently related, in reality belong to distinct types, and that, therefore, the necessity for presuming a connection between their distribution thus disappears. The Galapagos tortoises may be presumed to have come from the American continent; and as these animals can survive long exposure to sea, and are tenacious of life, their own conveyance or that of their eggs, on driftwood for example, is a hypothesis involving no great deniands upon a scientific imagination. The Mascarene tortoises may have similarly been conveyed from Africa; and there is no greater difficulty, therefore, in accounting for the detached existence of these great reptiles, than in explaining how their more diminutive kith and kin, belonging, like the giant tortoises, to different groups, have acquired such an extensive range over the earth's surface. Indeed, the case of the tortoises may serve to remind us of that of Bassaris, an animal formerly regarded as a kind of weasel or civet, but shown conclusively by Professor Flower to belong to the racoons of the New World. Bassaris, however, inhabits California, Texas, and Upper Mexico, and when it was regarded as a "civet” (Viverrila), an anomaly at once arose, since all known “civets” inhabit the Old World. But when the supposed “civet" was proved to be a member of the racoon group, all the difficulties of the case vanished ; inasmuch as, being one of the Procyonida or racoons, it fell naturally into its habitat, since all the members of this family are limited in their distribution to the New World. An error in classification may thus generate anomalies in distribution which further research proves to have no real existence.
These illustrations of the manner in which the difficulties of distribution are resolved may serve to show besides the wide demands which this science makes upon well-nigh every department of natural science. The issues of distribution, in fact, involve an acquaintance with the entire range of not merely biological study but of geological investigation as well ; whilst the deductions of distributional science, more perhaps than those of any other department of biology, open up before us the widest possible vista of human knowledge, and link together the varied interests of workers in every field of natural-science study. Nor is it in the grander aspects of this science that its far-reaching extent is alone to be seen. Even the apparently trivial details that constitute the story of the life existing on a barren and desolate islet may play an important part in the solution of questions dealing with the nature of life in its highest grades. And thus availing itself of knowledge from every source, this department of biology, more forcibly perhaps, as a whole, than any other branch of life-science, demonstrates how the true history of the existing universe is a history of variation and change-a chronicle, whereof the materials for each fresh chapter are derived from the lessons and the teachings of both the remote and the recent past.