Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/304

286 D I S T 11 I B U T I O N [ VKGTCTARLE. It thus appears that the general geological principle with which we started, of the more complete destruction by denudation and metamorphism of the earlier as compared with the later records of life upon the earth, receives ample support in the apparently sudden appearance of whole groups of complex and specialized forms in some of the earliest rocks ; while the general imperfection of the geo logical record is made manifest by such facts as the very few and isolated remains of Mammalia in the Mesozoic rocks, although we know they must have existed in abun dance throughout the whole Secondary and much of the Palaeozoic periods. The great lesson we have to learn from the facts of palaeontology is, that its negative evidence is at the best of but little value ; but when this negative evidence is opposed to general principles established upon a wide basis of physical and biological research, it becomes absolutely worthless. Just in proportion as the series of fossiliferous deposits is more complete, and the fossil remains more abundant and varied, does the evidence for evolution and progressive development become more powerful. The difficulties are almost wholly dependent on incomplete knowledge, and on the assumption (which we have endeavoured to show is entirely unfounded) that the earliest traces of the fossil remains of any animal type which have been discovered can give, even approximately, the period of its first appearance upon the earth. We find, then, that just as a study of the distribution of animals in space enables us to learn much of the immedi ately preceding condition of the earth s surface, and especially of the recent changes of land and water, so a study of tho distribution of animals in time, when aided by the modern theory of evolution, gives us some knowledge of the physical condition and life of the earth in times beyond the reach even of geological history. (A. K. w.) DISTRIBUTION OF VEGETABLE LIFE. The literature in which the immense multitude of dis tinct kinds of plants which are dispersed over the earth s surface and form its vegetation has so far been described has necessarily been adapted to the divisions of political geography. The causes which have brought about the formation of such divisions have rarely, however, had any thing in common with those which have determined the characteristic features, whether superficial or profound, of the floras of different countries. The great mass of catalogues and descriptive enumerations of the plants of such countries, the boundaries of which are for the most part quite artificial, are therefore ill adapted for bringing out any general conclusions as to the mode in which plants are distributed. It is only by making some kind of analysis of the often heterogeneous contents of such catalogues, and piecing together the results obtained from different sources, that any clue can be obtained to the approximate lines of demarcation of floras which are really naturally limited and characterized. The process is, however, enormously laborious, and, even apart from that, must for a long time to come be exceedingly imperfect in its application, owing to the immense tracts of land and those with the most varied and copious vegetation of the natural products of which our knowledge is still most defective. Numerous attempts have, however, been made, notwith standing the difficulty of the task, to map out the earth s .surface into &quot; regions of vegetation.&quot; The real significance of these regions will, of course, entirely depend upon the principles which have been relied upon in forming them. And in this respect the progress of geographical botany has been exactly similar to that of classification. The characteristic distinctions which were first seized upon in either case proved on closer scrutiny to be superficial, and to bring about merely artificial and arbitrary assemblages. The doctrine of evolution has in fact effected the same revolution in both ; it has shown in the one that community of dessent is the real meaning of a natural classification, it has shown in the other that community of origin is the real key to geographical distribution. Most of the writers on geographical botany have been content to set aside all considerations of origin and history in attempting to define the limits of botanical regions. They have not attempted to see in the peculiar features which such regions may possess anything more than adaptations to physical conditions working on plants created in great measure where they are found. Although, therefore, the literature of geographical botany has been useful in enabling the reader to realize the local features the colouring, if one may so express it of particular countries, the facts have hitherto been presented in a form void of any true significance. And these remarks apply to the system of Schouw (1833), which has been much employed, partially to that of De Candolle, and conspicuously to that more recently published by Grisebach. It is to the writings of Darwin, Hooker, Asa Gray, and Bentham that we must look for a real insight into the origin and dispersion of floras, and for the real causes of the existing distribution of plant life. The first attempt to review the whole subject of plant- distribution from the modern point of view afforded by evolution is due to Bentham, who made it the sub ject of a presidential address delivered to the Linnean Society in 1869. Bentham s conclusions are based upon the experience of a long life devoted to systematic botany, and will probably always hold a fundamental position in the study of the subject ; at any rate for some time to come, until the distribution of a large number of sub ordinate groups has been carefully worked out, the main points established by him are not likely to be materially modified. The general fades of vegetation is obviously largely affected by purely physical causes. In the polar regions, arboreal and even shrubby plants become incapable of existence, and only small perennials which are safely covered up by snow during the long winter are able in the brief summer to expand their flowers and ripen their seeds. Putting aside for the moment the severances effected by large bodies of water and mountain chains, it is easy to see that the vegetation of the earth must have always been separable into three great latitudinal zones, two belonging to the north and south hemispheres respec tively, and one dividing them lying between the tropics. The constituents of the vegetation of these zones must always have had a certain homogeneity ; very considerable divergences, however, have grown up within the zones themselves, owing to circumstances of geographical isolation. Even without these, distance alone, independently of isola tion, would in time be sufficient to effect it. It is also obvious that the precise northern and southern limitations of such hypothetical zones must have varied with secular changes in the earth s climate, and when these changes have taken place over a broken configuration of land and sea, the intermixture of diverse floras must necessarily have become very complicated. Underlying, however, the tangled fabric of the earth s existing floral covering, we may agree with Bentham 1 in recognizing the existence of three tolerably ancient floras the Northern, the Tropical, and the Southern. I. The Northern is characterized by its needle-leaved Couiferte, its catkin-bearing Amentacece and other forest 1 Presidcutal adcbvss to Linnean Society, 1869, p. 18.