Page:EB1911 - Volume 28.djvu/1030

GENERAL] would, for the most part at any rate, be universally true. Geology has, however, taught us that many parts of what are now continents formed at earlier periods of the earth's history portions of the ocean-bed, while what are now islands have in some instances been connected with the adjacent mainlands, or even with land-masses the sites of which are now occupied by the open sea.

We can hope, therefore, to understand and explain the present distribution of terrestrial animal life only by taking into account what geology teaches us as to past changes in the configuration of the land-masses of the globe, accompanied by investigations into the past history of animals themselves, as revealed by their fossil remains.

Although to understand the reason of many facts in the present distribution of animals—as, for example, why tapirs are confined to the Malay countries and South America—it is essential to study fossil faunas, yet it has been found possible from the consideration of existing faunas alone to map out the land-surface of the globe into a number of zoological "regions," or provinces, more or less independent of the ordinary geographical boundaries, and severally characterized by a greater or smaller degree of distinctness in the matter of their faunas. One of the pioneers in this line of research was Dr P. L. Sclater, who in a paper on the geographical distribution of birds, published in the Journal of the Linnean Society of London for 1858, was enabled to define and name six of such zoological regions; these being mainly based on the distribution of the perching or passerine birds. Two years later Dr A. Russel Wallace, in the same journal, discussed in some detail the problems presented by the distribution of animals in the Malay Archipelago and Australasia. This preliminary essay was followed in 1876 by the appearance of the latter author's Geographical Distribution of Animals, an epoch-making work, which may be said to have first put the study of the distribution of animals generally on a thoroughly firm and scientific basis. With some slight modifications, the names proposed for the six zoological regions by Dr Sclater were adopted by Dr Wallace. Certain changes in regard to the limits and number of the zoological regions adopted by Sclater and Wallace have been proposed; but the original scheme forms the basis of all the later modifications, and these eminent naturalists are entitled to be regarded as the fathers of the study of distributional zoology. T. H. Huxley was also one of those who did much to advance the science in its early days, while among those who have proposed more or less important modifications of the original scheme special mention may be made of Dr W. T. Blanford, Dr A. Heilprin, Prof. P. Matschie and Prof. Max Weber.

The zoological regions proposed by Dr Sclater were based mainly on the distribution of the perching birds; but in the writings of Dr Wallace and of later authors mammals were very largely taken into consideration, and in later schemes there has been a similarly extensive use of the evidence afforded by mammalian distribution. That different groups of animals do not agree with another in the matter of geographical distribution will be evident when we reflect that in many instances there are very great differences in the relative ages of such groups, or, at all events, in the dates of their dispersal, or "radiation," over the surface of the earth. The radiation and dominance of reptiles, for example, greatly antedated that of either birds or mammals. Consequently, the zoological regions indicated by the present geographical distribution of the former group are very different from those suggested by the distribution of the two latter. If zoological regions are based on the evidence of the existing distribution of animals, groups with a relatively late radiation are clearly to be preferred to those the dispersal of which was earlier. Mammals and birds, therefore, are of greater value from this point of view than reptiles; while the absence of the power of flight in the great bulk of the class renders the evidence afforded by mammals superior to that derived from birds. The marked general agreement between the geographical distribution of birds on the one hand and of mammals on the other is, however, a fact of the greatest importance in regard to the value of the zoological regions established on their evidence. Further testimony in the same direction is afforded by the distribution of certain other groups, more especially spiders (Arachnida); and it is also noteworthy that the distribution of the three main divisions of the human race accords to a certain extent with the boundaries of some of the zoological regions based on the distribution of the lower animals.

With regard to the theory of the polar origin of life and the gradual dispersal of animals from the arctic regions, it may be briefly stated that the presumed series of radiations of life southward from the northern pole can have nothing to do with the present geographical distribution of animals, since we have abundant evidence that mammals have been spread over the whole of the warmer parts of the globe since, at any rate, the commencement of the Tertiary period, while the radiation of reptiles commenced at a much earlier epoch.

As regards barriers to the free dispersal of nonvolant terrestrial animals these may be grouped under two main heads, namely, climatic and geographical, of which the second is by far the more important. At the present day a certain number of animals are fitted to live respectively only in hot and in cold climates. The man-like apes and elephants among mammals, and trogons and parrots among birds, are, for example, now exclusively dwellers in tropical or subtropical climates; whereas the polar bear, the musk-ox and ptarmigan are equally characteristic of the arctic zone. To a great extent this must be regarded as a comparatively modern adaptive feature, since many of these arctic and tropical animals belong to groups the distribution of which, either in the past or the present, is more or less independent of climate. Elephants, for instance, formerly inhabited Siberia at a time when the climate, although probably less cold than at present, was certainly not tropical; while the polar bear is a specialized member of a group some of the representatives of which are denizens of the tropics.

It is true, indeed, that within the limits of the different zoological regions temperature-control has had an important influence on the distribution of animals, and has resulted in certain cases in the formation of life-zones, as in North America. As remarked, however, by H. A. Pilsbry and J. H. Ferriss in connexion with, the distribution of land-molluscs, "the life- zones of the United States as mapped by Dr C. H. Merriam emphasize the secondary and not the primary facts of distribution. The laws of temperature-control do not define transcontinental zones of primary import zoologically. These zones are secondary divisions of vertical life-areas of which the molluscan faunas were evolved in large part independently." And what is true of molluscs will hold good in the case of several other groups.