Online Encyclopedia

DISTRIBUTION OF

Online Encyclopedia
Originally appearing in Volume V21, Page 783 of the 1911 Encyclopedia Britannica.
Spread the word: del.icio.us del.icio.us it!
DISTRIBUTION OF PLANTS Common experience shows that temperature is the most important condition which controls the distribution of plants. Those of warmer countries cannot be cultivated in British gardens without protection from the rigours of winter; still less are they able to hold their own unaided in an unfavourable climate. Temperature, then, is the fundamental limit which nature opposes to the indefinite extension of any one species. Buffon remarked " that the same temperature might have been expected, all other circumstances being equal, to produce the same beings in different parts of the globe, both in the animal and vegetable kingdoms." Yet lawns in the United States are destitute of the common English daisy, the wild hyacinth of the woods of the United Kingdom is absent from Germany, and the foxglove from Switzerland. We owe to Buffon the recognition of the limitation of groups of species to regions separated from one another by " natural barriers." When by the aid of man they surmount these, they often dominate with unexpected vigour the native vegetation amongst which they are colonists. The cardoon and milk thistle, both European plants, cover tracts of country in South America with impenetrable thickets in which both man and beast may be hopelessly lost. The watercress blocks the rivers of New Zealand into which it has been introduced from Europe. The problem, then, which plant-distribution presents is twofold: it has first to map out the earth's surface into " regions " or " areas of vegetation," and secondly to trace the causes which have brought them about and led to their restriction and to their mutual relations. The earliest attempts to deal with the first branch of the inquiry may be called physiognomical. They endeavoured to define " aspects of vegetation " in which the " forms " exhibited an obvious adaptation to their climatic surroundings. This has been done with success and in great detail by Grisebach, whose Vegetation der Erde from this point of view is still unsurpassed. With it may be studied with advantage the unique collection at Kew of pictures of plant-life in its broadest aspects, brought together by the industry and munificence of Miss Marianne North. Grisebach declined to see anything in such " forms " but the production by nature of that which responds to external conditions and can only exist as long as they remain unchanged. We may agree with Schimper that such a point of view is obsolete without rejecting as valueless the admirable accumulation of data of which it admittedly fails to give any rational explanation. A single example will be sufficient to illustrate this. The genus Senecio, with some loco species, is practically cosmopolitan. In external habit these exhibitadaptations to every kind of climatic or physical condition: they may be mere weeds like groundsels or ragworts, or climbers masquerading like ivy, or succulent and almost leafless, or they may be shrubs and even trees. Yet thrcughout they agree in the essential structure of their floral organs. The cause of such agreement is, according to Grisebach, shrouded in the deepest obscurity, but it finds its obvious and complete explanation in the descent from a common ancestor which he would unhesitatingly reject. From this point of view it is not sufficient, in attempting to map out the earth's surface into " regions of vegetation, " to have regard alone to adaptations to physical conditions. We are compelled to take into account the actual affinity of the plants inhabiting them. Anything short of this is merely descriptive and empirical, and affords no rational basis for inquiry into the mode in which the distribution of plant-life has been brought about. Our regions will not be " natural " unless they mark out real discontinuities both of origin and affinity, and these we can only seek to explain by reference to past changes in the earth's history. We arrive thus at " the essential aim of geographical botany," which, as stated by Schimper, is " an inquiry into the causes of differences existing among the various floras." To quote further: " Existing floras exhibit only one moment in the history of the earth's vegetation. A transformation which is sometimes rapid, some-times slow, but always continuous, is wrought by the reciprocal action of the innate variability of plants and of the variability of the external factors. This change is due partly to the migrations of plants, but chiefly to a transformation of the plants covering the earth." This transformation is due to new characters arising through variation. " If the new characters be useful, they are selected and perfected in the descendants, and constitute the so-called `adaptations' in which the external factors acting on the plants are reflected." The study of the nature of these adaptations, which are often extremely subtle and by no means merely superficial, is termed Ecology (see above). The remark may conveniently find its place here that plants which have reached a high degree of adaptive specialization have come to the end of their tether: a too complicated adjustment has deprived them of the elasticity which would enable them to adapt themselves to any further change in their surroundings, and they would pass away with conditions with which they are too inextricably bound up. Vast floras have doubtless thus found their grave in geologic change. That wrought by man in destroying forests and cultivating the land will be no less effective, and already specimens in our herbaria alone represent species no longer to be found in a living state. Extinction may come about indirectly and even more surely. This is easy to happen with plants dependent on insects for their fertilization. Kronfeld has shown that aconites are dependent for this on the visits of a Bombus and cannot exist outside the area where it occurs. The actual and past distribution of plants must obviously be controlled by the facts of physical geography. It is concerned with the land-surface, and this is more symmetrically disposed than would at first sight appear from a glance at a map of the world. Lyell points out that the eye of an observer placed above a point between Pembroke and Wexford, lat. 52° N. and long. 6° W., would behold at one view the greatest possible quantity of land, while the opposite hemisphere would contain the greatest quantity of water. The continental area is on one side of the sphere and the oceanic on the other. Love has shown (Nature, Aug. 1, 1907, p. 328) that this is the result of physical causes and that the existence of the Pacific Ocean " shows that the centre of gravity of the earth does not coincide with the centre of figure." One half of the earth has therefore a greater density than the other. But " under the influence of the rotation the parts of greater density tend to recede further from the axis than the parts of less density . . . the effect must be to produce a sort of furrowed surface." The furrows are the great ocean basins, and these would still persist even if the land surface were enlarged to the 1400 fathoms contour. These considerations preclude the possibility of solving difficulties in geographical distribution by the construction of hypothetical land-surfaces, an expedient which Darwin always stoutly opposed (Life and Letters, ii. 74-78). The furrowed surface of the earth gives the land-area a star-shaped figure, which may from time to time have varied in outline, but in the main has been permanent. It is excentric as regards the pole and sends tapering extensions towards the south. Sir George Darwin finds a possible explanation of these in the screwing motion which the earth would suffer in its plastic state. The polar regions travelled a little from west to east relatively to the equatorial, which lagged behind. The great primary divisions of the earth's flora present them-selves at a glance. The tropics of Cancer and Capricorn cut off with surprising precision (the latter somewhat less so) the tropical from the north and south temperate zones. The north temperate region is more sharply separated from the other two than the south temperate region from the tropical. I. NORTH TEMPERATE REGION (Holarctic).—This is the largest of all, circumpolar, and but for the break at Bering Straits, would be, as it has been in the past, continuous in both the old and new worlds. It is characterized by its needle-leaved Coniferae, its catkin-bearing (Amentaceae) and other trees, deciduous in winter, and its profusion of herbaceous species. II. SOUTH TEMPERATE REGION.—This occupies widely separated areas in South Africa, Australia, New Zealand and South America. These are connected by the presence of peculiar types, Proteaceae, Restiaceae, Rutaceae, &c., mostly shrubby in habit and on the whole somewhat intolerant of a moist climate. Individual species are extremely numerous and often very restricted in area. A consideration of these regions makes it apparent that they are to a large extent adaptive. The boreal is cold, the austral warm, and the tropical affords conditions of heat and moisture to which the vegetation of the others would be intolerant. If we take with Drude the number of known families of flowering plants at 240, 92 are generally dispersed, 17 are more restricted, while the remainder are either dominant in or peculiar to separate regions. Of these 40 are boreal, 22 austral and 69 tropical. If we add to the latter figure the families which are widely dispersed, we find that the tropics possess 161 or almost exactly two-thirds of the large groups comprised in the world's vegetation. M. Casimir de Candolle has made an independent investigation, based on Hooker and Bentham's Genera pientarum. The result is unfortunately (1910) unpublished, but he informs the present writer that the result leads to the striking conclusion: " La vegetation est un phenomene surtout intertropical, dont nous ne voyons plus que restes affaiblis dans nos regions teinperees." In attempting to account for the distribution of existing vegetation we must take into account palaeontological evidence. The results arrived at may be read as a sequel to the article on PALAEOBOTANY. The vegetation of the Palaeozoic era, till towards its close, was apparently remarkably homogeneous all over the world. It was characterized by arborescent vascular Cryptogams and Gymnosperms of a type (Cordaiteae) which have left no descendants beyond it. In the southern hemisphere the Palaeozoic flora appears ultimately to have been profoundly modified by a lowering of temperature and the existence of glacial conditions over a wide area. It was replaced by the Glossopteris flora which is assumed to have originated in a vast continental area (Gondwana land), of which remnants remain in South America, South Africa and Australia. The Glossopteris flora gradually spread to the northern hemisphere and intermingled with the later Palaeozoic flora which still persisted. Both were in turn replaced by the Lower Mesozoic flora, which again is thought to have had its birth in the hypothetical Gondwana land, and in which Gymnosperms played the leading part. formerly taken by vascular Cryptogams. The abundance of Cycadean plants is one of its most striking features. They attained the highest degree of structural complexity in the Bennettiteae, which have been thought even to foreshadow a floral organization. Though now on the way to extinction, Cycadeae are still widely represented in the southern hemisphere by genera which, however, have no counterpart in the Mesozoic era. Amongst Conifers the archaic genera, Ginkgo and Araucaria still persist. Once widely distributed in the Jurassic period throughout the world, they are now dying out: the former is represented by the solitary maiden-hair tree of China and Japan; the latter by some ten species confined to the southern hemisphere, once perhaps their original home. With them may be associated the anomalous Sciadopitys of Japan. So far the evolution of the vegetable kingdom has proceeded with-out any conspicuous break. Successive types have arisen in ascending sequence, taken the lead, and in turn given way to others. Butthe later period of the Mesozoic era saw the almost sudden advent of a fully developed angiospermous vegetation which rapidly occupied the earth's surface, and which it is not easy to link on with any that preceded it. The closed ovary implies a mode of fertilization which is profoundly different, and which was probably correlated with a simultaneous development of insect life. This was accompanied by a vegetative organization of which there is no obvious foreshadowing. As Clement Reid remarked: " World-wide floras, such as seem to characterize some of the older periods, have ceased to be, and plants are distributed more markedly according to geographical provinces and in climatic zones." The field of evolution has now been transferred to the northern hemisphere. Though Angiosperms become dominant in all known plant-bearing Upper Cretaceous deposits, their origin dates even earlier. In Europe Heer's Populus primaeva from the Lower Cretaceous in Greenland was long accepted as the oldest dicotyledonous plant. Other undoubted Dicotyledons, though of uncertain affinity, of similar age have now been detected in North America. The Cenomanian rocks of Bohemia have yielded remains of a sub-tropical flora which has been compared with that existing at present in Australia. Upper Cretaceous formations in America have yielded a copious flora of a warm-temperate climate from which it is evident that at least the generic types of numerous not closely related existing dicotyledonous trees had already come into existence. It may be admitted that the identification of fragmentary leaf-remains is at most precarious. Even, however, with this reservation, it is difficult to resist the mass of evidence as a whole. And it is a plausible conjecture that the vegetation of the globe had already in its main features been constituted at this period much as it exists at the present moment. There were oaks, beeches (scarcely distinguishable from existing species), birches, planes and willows (one closely related to'the living Salix candida), laurels, represented by Sassafras and Cinnamomum, magnolias and tulip trees (Liriodendron), myrtles, Liquidambar, Diospyros and ivy. This is a flora which, thinned out by losses, practically exists to this day in the southern United States. And one essentially similar but adapted to slightly cooler conditions existed as far north as the latitude of Greenland. The tertiary era opens with a climate in which during the Eocene period something like existing tropical conditions must have obtained in the northern hemisphere. The remains of palms (Sabal and Nipa) as well as of other large-leaved Monocotyledons are preserved. Sequoia (which had already appeared in the American Upper Cretaceous) and the deciduous cypress (Taxodium distichum) are found in Europe. Starkie Gardner has argued with much plausibility that the Tertiary floras which have been found in the far north must have been of Eocene age. That of Grinnell Land in lat. 81 ° consisted of Conifers (including the living spruce), poplars and willows, such as would be found now 25° to the south. The flora of Disco Island in lat. 700 contained Sequoia, planes, maples and magnolias, and closely agrees with the Miocene flora of central Europe. Of this copious remains have been found in Switzerland and have been investigated with great ability by O. Heer. They point to cooler conditions in the northern hemisphere: palms and tropical types diminish; deciduous trees increase. Sequoia and the tulip-tree still remain; figs are abundant; laurels are represented by Sassafras and camphor; herbaceous plants (Ranunculaceae, Cruciferae, Umbelliferae) are present, though, as might be expected, only fragmentarily preserved. We may draw with some certainty the conclusion that a general movement southward of vegetation had been brought about. While Europe and probably North America were occupied by a warm temperate flora, tropical types had been driven southward, while the adaptation of others to arctic conditions had become accentuated. A gradual refrigeration proceeded through the Pliocene period. This was accompanied in Europe by a drastic weeding out of Miocene types, ultimately leaving the flora pretty much as it now exists. This, as will be explained, did not take place to anything like the same extent in North America, the vegetation of which still pre-serves a more Miocene facies. Torreya, now confined to North America and Japan, still lingered,- as did Ocotea, now profusely developed in the tropics, but in north temperate regions only existing in the Canaries: the evergreen oaks, so characteristic of the Miocene, were reduced to the existing Quercus ilex. At the close of the Pliocene the European flora was apparently little different from that now existing, though some warmer types such as the water-chestnut (Trapa natans) had a more northern extension. The glacial period effected in Europe a wholesale extermination of temperate types accompanied by a southern extension of the arctic flora. But Its operation was in great measure local. The Pliocene flora found refuges in favoured localities from which at its close the lowlands were restocked while the arctic plants were left behind on the mountains. During the milder interglacial period some southern types, such as Rhododendron ponticum, still held their own, but ultimately succumbed. The evidence which has thus been briefly summarized, points unmistakably to the conclusion that existing vegetation originated in the northern hemisphere and under climatic conditions corresponding to what would now be termed sub-tropical. It occupied a continuous circumpolar area which allowed free communication between the old and new worlds. The gradual differentiation of their floras has been brought about rather by extermination than specialization, and their distinctive facies by the development and multiplication of the surviving types. The distribution of mountain barriers in the Old and New Worlds is in striking contrast. In the former they run from east to west; in the latter from north to south. In the Old World the boreal zone is almost sharply cut off and afforded no means of escape for the Miocene vegetation when the climate became more severe. Thus in the Mediterranean region the large groups of palms, figs, myrtles and laurels are each only represented by single surviving species. The great tropical family of the Gesneraceae has left behind a few outliers: Ramondia in the Pyrenees, Haberlea in the Balkans, and Jankaea in Thessaly; the Pyrenees also possess a minute Dioscorea, sole European survivor of the yams of the tropics. In North America there is no such barrier: the Miocene flora has been able to escape by migration the fluctuations of climate and to return when they ameliorated. It has preserved its characteristic types, such as Magnolia, Liriodendron, Liquidambar, Torreya, Taxodium and Sequoia. While it has been customary to describe the Miocene flora of Europe as of a North American type, it would be more accurate to describe the latter as having in great measure preserved its Miocene character. If mountains serve as barriers which arrest the migration of the vegetation at their base, their upper levels and summits afford lines of communication by which the floras of colder regions in the northern hemisphere can obtain a southern extension even across the tropics. They doubtless equally supply a path by which southern temperate types may have extended northwards. Thus the characteristic assemblage of plants to which Sir Joseph Hooker has given the name Scandinavian " is present in every latitude of the globe, and is the only one that is so " (Trans. Linn. Soc. xxiii. 253). In the mountains of Peru we find such characteristic northern genera as Draba, Alchemilla, Saxifraga, Valeriana, Gentiana and Bartsia. High elevations reproduce the physical conditions of high latitudes. The aqueous vapour in the atmosphere is transparent to luminous but opaque to obscure heat-rays. The latter are retained to warm the air at lower levels, while it remains cold at higher. It results that besides a horizontal distribution of plants, there is also an altitudinal: a fact of cardinal importance, the first observation of which has been attributed to Tournefort. Speaking generally, all plants tend to exhaust particular constituents of the soil on which they grow. Nature therefore has provided various contrivances by which their seeds are disseminated beyond the actual position they occupy. In a large number of cases these only provide for migration within sufficient but narrow limits; such plants would be content to remain local. But other physical agencies come into play which may be briefly noticed. The first of these is wind: it cannot be doubted that small seeds can be swept I herbacea, Silene acaulis and Dryas octopetala will serve as examples. up like dust and transported to considerable distances. This is Even so small an area as that of Britain illustrates what has already certainly the case with fern-spores. The vegetation of Krakatoa been pointed out, that the species of a flora change both with latitude and altitude. Watson further brought out the striking fact that the west and east of Britain each had species peculiar to it; the former he characterized as Atlantic, the latter as Germanic. The Cornish heath (Erica vagans) and the maiden-hair fern (Adiantum Capillus-Veneris) may serve as instances of the one, the man-orchis (Aceras anthropophora) and Reseda lutea of the other. Ireland illustrates the same fact. It possesses about moo species, or about two-thirds the number of Britain. On its western shores there are some twenty, such as Saxifraga umbrosa, Erica mediterranea and Arbutus unedo, which are not found in Britain at all. The British Phanerogamic flora, it may be remarked, does not contain a single endemic species, and 38 % of the total number are common to the three northern continents. The analysis of larger areas yields results of the same kind. Within the same region we may expect to find considerable differences as we pass from one meridian to another. Assuming that in its circumpolar origin the North Temperate flora was fairly homogeneous, it would meet in its centrifugal extension with a wide range of local conditions; these would favour the preservation of numerous species in some genera, their greater or less elimination in others. Thus comparing the Nearctic and Palaearctic floras we find striking differences overlying the points of agreement already indicated. The former is poor in Cruciferae, Caryophyllaceae, Umbelliferae, Primulaceae and Labiatae; but for the occurrence of Calluna in Newfound-land it would have no heaths. On the other hand, it is rich in Compositae, especially Solidago and Aster, Polemoniaceae, Asclepiadaceae, Hydrophyllaceae and Cyperaceae, and it has the endemic Sarracenia, type of a family structurally allied to poppies, of which of the remaining genera Darlingtonia is Californian, and Heliamphora Venezuelan. These distinctions led Sir Joseph Hooker to claim for the two divisions the rank of primary regions. Darwin doubted, however, whether they ought to be separated (Life, iii. 230). Lyell, discussing the facts of zoological distribution, admits that " the farther we go north . . . the more the discordance in genera and species diminishes " (Principles, ii. 340); and Hemsley finds that not less than 75 % of the genera in the flora of eastern North America " are represented in the old world," and, according to Asa Gray, 50 % in Europe. Latitudinally the region subdivides naturally into several well-marked sub-regions which must be briefly discussed. as in the Old World—oaks, chestnuts, beeches, hazels, hornbeams, birches, alders, willows and poplars. Or to take the small but well-defined group of five-leaved pines, all the species of which may be seen growing side by side at Kew under identical conditions: we have the Weymouth pine (Pinus Strobus) in eastern North America, P. monticola and the sugar pine (P. Lambertiana) in California, P. Ayacahuste in Mexico, the Arolla pine (P. Cembra) in Switzerland and Siberia, P. Pence in Greece, the Bhotan pine (P. excelsa) in the Himalayas, and two other species in Japan. Amongst broad-leaved trees Juglans has a similar Holarctic range, descending to the West Indies; so has Aesculus, were it not lacking in Europe; it becomes tropical in South America and Malaya. If we turn to herbaceous plants, Hemsley has pointed out that of the thirteen genera of Ranunculaceae in California, eleven are British. While the tropics preserve for us what remains of the pre-Tertiary or, at the latest, Eocene vegetation of the earth, which formerly had a much wider extension, the flora of the North Temperate region is often described as the survival of the Miocene. Engler therefore calls it Arcto-Tertiary. We must, however, agree with Starkie Gardner that it is only Miocene as regards its present position, which was originally farther north, and that its actual origin was much earlier. There has been in effect a successive shifting of zones of vegetation southwards from the pole. Their distinctive and adaptive characteristics doubtless began to be established as soon as the phanerogamic flora was constituted. There is no reason to suppose that the peculiarities of the arctic flora are more modern than those of any other, though there is no fossil evidence to prove that it was not so. The North Temperate region admits of subdivision into several well-marked sub-regions. The general method by which this is effected in this and other cases is statistical. As A. de Candolle, however, points out, exclusive reliance on this may be misleading unless we also take into account the character and affinities of the plants dealt with (Geogr. Bot. i. 1166). The numerical predominance of certain families or their absence affords criteria for marking out boundary lines and tracing relationships. The analysis of the flora of the British Isles will afford an illustration. This was first attempted in 1835 by H. C. Watson, and his conclusions were en-forced ten years later by Edward Forbes, who dealt also with the fauna. Watson showed that Scotland primarily, and to a less extent the north of England, possessed species which do not reach the south. Such are the crowberry (Empetrum nigrum), Trientalis europaea, Rubus saxatilis and the globe-flower (Trollius europaeus). He further found that there was an element which he termed " boreal in a more intense degree," which amounted to about " a fifteenth of the whole flora." This was not confined to the north .but may occur on the mountains of England and Wales: Salix was completely exterminated in 1883 by a thick coat of red-hot pumice. Yet in 1886 Treub found that it was beginning to cover itself again with plants, including eleven species of ferns; but the nearest source of supply was to m. distant. Seeds are carried with more facility when provided with plumes or wings. Treub found on Krakatoa four species of composites and two grasses. Water is another obvious means of transport. The littoral vegetation of coral islands is derived from sea-borne fruits. The seeds of West Indian plants are thrown on the western shores of the British Isles, and as they are capable of germination, the species are only pre-vented from establishing themselves by an uncongenial climate. Travers picked up a seed of Edwardsia in the Chatham Islands, evidently washed ashore from New Zealand (Linn. Soc. Journ. ix. 1865). Rivers bring down the plants of the upper levels of their basins to the lower: thus species characteristic of the chalk are found on the banks of the Thames near London. Birds are even more effective than wind .in transporting seeds to long distances. Seeds are carried in soil adhering to their feet and plumage, and aquatic plants have in consequence for the most part an exception-ally wide range. Fruit-pigeons are an effective means of transport in the tropics by the undigested seeds which they void in their excrement. Quadrupeds also play their part by carrying seeds or fruits entangled in their coats. Xanthium spinosum has spread from the Russian steppes to every stock-raising country in the world, and in some cases has made the industry impossible. Even insects, as in the case of South African locusts, have been found to be a means of distributing seeds. The primary regions of vegetation, already indicated, and their subordinate provinces may now be considered more in detail. I. NORTH TEMPERATE REGION.—Many writers on the distribution of animals prefer to separate this into two regions of " primary rank " : the Palaearctic and the Nearctic. But to justify such a division it is necessary to establish either an exclusive possession or a marked predominance of types in the one which are correspondingly deficient in the other. This cannot apparently be done for insects or for birds; Newton accordingly unites the two into the Holarctic region. It equally fails for plants. To take, for example, one of the most characteristic features of the Palaearctic region, its catkin-bearing deciduous trees: in North America we find precisely the same genera 1. The Arctic-Alpine sub-region consists of races of plants belonging originally to the general flora, and recruited by subsequent additions, which have been specialized in low stature and great capacity of endurance to survive long dormant periods, sometimes even unbroken in successive years by the transitory activity of the brief summer. It is continuous round the pole and roughly is bounded by the arctic circle. Mature seeds are highly tolerant of cold and have been shown to be capable of withstanding the temperature even of liquid hydrogen. Arctic plants make their brief growth and flower at a temperature little above freezing point, and are dependent for their heat on the direct rays of the sun. Characteristic representatives are Pa paver nudicaule, Sazifraga oppositifolia, which forms a profuse carpet, and Dryas octopetala. Such plants perhaps extend to the most northern lands at present known. On May 3oth, in Ward Hunt's Island, lat. 83° 5', Sir George Nares found that vegetation was fairly represented as regards quantity in the poppy, saxifrage and small tufts of grass." We may compare this with extreme alpine conditions: on a spot above the Aletsch glacier at a height of 10,700 ft. Ball found the temperature one inch below the surface to be 83°, and he collected " over forty species in flower." Taking the whole arctic flora at 762 species, Hooker found that 616 occurred in arctic Europe, and of these 586 are Scandinavian. Beyond the arctic circle some 200, or more than a quarter, are confined to the mountains of the northern hemisphere and of " still more southern regions." This led Hooker to the striking observation already quoted. The arctic flora contains no genus that is peculiar to it, and only some fifty species that are so. Christ has objected to terming the arctic flora Scandinavian, but the name implies nothing more than that Scandinavia has been its chief centre of preservation. A detailed examination of mountain floras shows that a large local element is present in each besides the arctic. The one is in fact the result of similar physical conditions to that which has produced the other. Thus Sazifraga cernua is regarded as an alpine form of the lowland S. granulate. Comparing the Alps with the Pyrenees, according to Ball, each has about half its flora common to the other: " the Alps have 172 endemic species and at least 15 genera that are not found in the Pyrenees, while the latter range counts about loo endemic species with several (six or seven) genera not found in the Alps." Drude has accordingly suggested the substitution of the term " High-mountain floras " for Alpine, which he regards as misleading. Its meaning has, however, become synonymous and is consecrated by usage. The repetition of the same species in the arctic regions and in the high mountains of the North Temperate region is generally attributed to the exchange which took place during the glacial period. This was first suggested by Edward Forbes in 1846, though the idea had earlier suggested itself to Darwin (Life, i. 88). It took place south-wards, for the arctic flora is remarkably uniform, and, as Chodat points out, it shows no evidence of having been recruited from the several mountain floras. That the arctic flora was driven south into Central Europe cannot be contested in the face of the evidence collected by Nathorst from deposits connected with the boulder-clay. And Reid has shown that during the glacial period the existing flora was replaced by an arctic one represented by such plants as Salix polaris, S. herbacea, S. reticulata and Betula nana. At the same time the then existing alpine floras descended to lower levels, though we may agree with Ball that they did not necessarily become extinct at higher ones as long as any land-surface remained uncovered by ice. At the close of the glacial period the alpine floras retreated to the mountains accompanied by an arctic contingent, though doubtless many species of the latter, such as Salix polaris, failed to establish themselves. Christ, while admitting an ancient endemic element, such as Campanula excisa in the arctic-alpine flora of Europe, objects that a Scandinavian colonization could not furnish such characteristic plants as the larch and edelweiss. He traces the original home of the bulk of existing alpine plants to northern Asia. the mountains of which appear to have escaped glaciation. At the close of the glacial epoch the north Asiatic flora spread west-wards into Europe and intermingled with the surviving vegetation. Some species, such as Anemone alpina, which are wanting in the Arctic flora of the Old World, he thinks must have reached Europe by way of Greenland from north-east America. 2. The Intermediate sub-region comprises the vegetation of the large area occupied by the steppes of the Old World, the prairies of the new and the forest region of both. The former support a copious herbaceous flora, the characteristics of which in the Old and New Worlds have been already briefly summarized. In the former that of Europe and of Central Asia are continuous. Of species common to the two, Maximowicz finds that Manchuria possesses 40% and scarcely 9 % that are endemic. Of a collection of about Soo species made in that country by Sir Henry James nearly a third are British. This confirms the theory of Christ that Europe was restocked mainly from Asia after the close of the glacial epoch, the south being closed to it. In the new world no southern barriers existed and it is more difficult to draw the line between contiguous sub-regions. The dominant characteristics of the arboreous vegetation are, besides deciduous and amentiferous trees, Coniferae, especially the more recent tribe of Abietineae—pines, silver-firs, hemlocks, spruces and larches, of which, unlike the older types, no representativecrosses the tropic. The prominent deciduous trees of Europe appear to be of eastern origin, and the progress of western migration has continued to historic times. The evidence of the peat bogs shows that the Scots fir, which is now extinct, was abundant in Denmark in the Roman period. It was succeeded by the sessile-fruited oak, which was in turn supplanted by the pedunculate form of the same tree. Quercus Robur has left no trace in the Tertiary deposits of Europe and it is most nearly allt2d to east Asiatic species. The oak in turn has been almost superseded in Denmark by the beech, which, if we may trust Julius Caesar, had not reached Britain in his time, though it existed there in the pre-glacial period, but is not native in either Scotland or Ireland. Its eastern limit in Europe is a line from Konigsberg to the Caucasus; thence through China it is continued by varietal forms to Japan. It has a solitary representative in North America. Broadly speaking, the American portion of the sub-region consists of an Atlantic and Pacific forest area and an intervening non-forest one, partly occupied by the Rocky Mountains, partly by intervening plains. Its arboreal vegetation is richer both in genera and species than that corresponding to it in the Old World. Glacial elimination has been less severe, or rather there has been, at any rate on the Atlantic side, an unimpeded return of Miocene types. The Atlantic area has five magnolias, a tulip tree, an Anonacea (Asimina), two Ternstroemiaceae (Stuartia and Gordonia), Liquidambar, Vitis (the fox-grape, V. Labrusca, reappears in Japan), and others; an assemblage, as long ago pointed out by Asa Gray, which can only be paralleled in the Chino-Japanese region, another centre of preservation of Miocene types. All these are wanting in the Pacific area, though there are indications in its gold-bearing gravels that it once possessed them. On the other hand, the latter " is rich in coniferous types beyond any country except Japan " (A. Gray), but till we reach California these are boreal in type. The Atlantic flora has also numerous oaks and maples, signalized by their autumnal coloration. These were abundant in Tertiary Europe, as they are now in Japan, and represent perhaps a cooler element in the flora than that indicated above. The highlands of Central America and the West Indies have preserved a number of Chino-Japanese types—Bocconia, Deutzia, Abelie, &c.—not met with elsewhere in the New World. 3. The Mediterraneo-Oriental sub-region contrasts no less vividly with the Intermediate than the Arctic-Alpine. It includes the Azores and Canaries, the Mediterranean basin, northern Africa as far as the Atlas and Sahara, Asia Minor, Persia and the countries eastward as far as Sind, being bounded to the north by the mountains which run from the Caucasus to the Hindu-Kush. Its extreme richness in number of species (it comprises six-sevenths of the European flora) and the extremely restricted areas of many of them point to a great antiquity. The Mediterranean basin has been a centre of preservation of Miocene vegetation: the oleander is said to have been found in local deposits of even earlier age, and the holm oak (Quercus Ilex) is the living representative of a Miocene ancestor. Extensions of the flora occur southwards of the high mountains of tropical Africa; Adenoearpus, a characteristic Mediterranean genus, has been found on Kilimanjaro and 2000 m. distant on the Cameroons. Two British plants may be added which both reach North Africa: Sanicula europaea extends from Abyssinia to the Cameroons and southwards to Cape Colony and Madagascar; Sambucus Ebulus reaches Uganda. The Mediterranean, however, has apparently been a barrier to the southward passage of the arcto-alpine flora which is totally wanting on the Atlas. The vegetation of the sub-region is rich in shrubs: myrtle, bay, Cistus, Pistacia, Arbatus, heaths in its western portion, and the ground-palm (Chamaerops). It is even richer in more herbaceous plants tolerant of a hot summer; giant Umbelliferae (such as Ferula) are especially characteristic and yield gum-resins which have long been reckoned valuable. Of the moo known species of Astragalus it possesses Soo. Evergreen oaks and Conifers form the forests. Asia Minor has a Liquidambar. The Argan tree (A rgania Sideroxylon) , which forms forests in Morocco, is a remarkable survivor of a tropical family (Sapotaceae). Amongst Conifers Cedrus is especially noteworthy; it is represented by geographical races in the north-west Himalaya, in Syria, Cyprus and North Africa. This well-marked sub-region has a deeper interest than the botanical. It has been the cradle of civilization, and to it is due the majority of cultivated plants. Such are the date in Mesopotamia (a second species of Phoenix occurs in the Canaries) ; most European fruits, e.g. the vine, fig, mulberry, cherry, apricot, walnut; pulses, e.g. the bean, pea and lentil; pot-herbs, e.g. lettuce, endive, beet, radish, cress; cereals; and fodder plants such as lucerne and carob. 4. The Chino-Japanese sub-region.—Of the vegetation of China till recently very little has been known. In 1873 Elwes pointed out that the Himalayan avifauna extended into north-west China and established the Himalo-Chinese sub-region. Shortly afterwards the collections of Prejewalsky confirmed it for the flora. And we now know that, excluding the southern tropical area, it has the same character throughout the whole of China proper. We may therefore regard the Himalayan flora as a westward extension of the Chinese rather than the latter as a development of the former. Of four genera which Hooker singles out as the largest in Sikkim, in China Corydalis has 76 species, Saxifraga 58, Pedicularis 129, and Primula 77. Of Rhododendron there are 134 species. Upwards of 8000 species are known out of a probable total of not less than 12,00o, America or Malays. A peculiar feature in which tropical Africa and of these more than half are endemic. The number of species to a genus, 3, is only half that found in other large areas. This aggregation of genera and of endemic species is characteristic of the circumferential portions of the earth's land surface: the expansion of the one and the further advance of the other is arrested. The sub-region is probably sharply cut off from the Intermediate. Maximowicz finds that 40 % of the plants of Manchuria are common to Europe and Asia, but the proportion falls sharply to 16% in the case of Japan. Its connexion with the Mediterraneo-Oriental sub-region is still more remote. China has no Cistus or heath. only a single Ferula, while Astragalus is reduced to 35 species. There are two species of Pistacia and four of Liquidambar. The affinity to Atlantic North America is strongly marked as it has long been known to be in Japan. China has 66 species of Quercus, 35 of Vitis, 2 of Aesculus, 42 of Acer, 33 Magnoliaceae (including two species of Liriodendron), 12 Anonaceae, 71 Ternstroemiaceae (including the tea-plant), and 4 of Clethra, which has a solitary western representative in Madeira. Trachycarpus and Rhapis are characteristic palms, and Cycadeae are represented by Cycas. 5. The Mexico-American sub-region has as its northern boundary the parallel of lat. 36° as far as New Mexico and then northwards to the Pacific coast at lat. 40°. The eastern and western halves are contrasted in climate--the former being moist and the latter dry—and have been distinguished by some zoologists as distinct sub-regions. They are in fact in some degree comparable to sub-regions 3 and 4 in the Old World. The absence of marked natural boundaries makes any precise north and south limitation difficult. But it has been a centre of preservation of the Taxodieae, a tribe of Coniferae of great antiquity. Taxodium (with single species in China and Mexico) is represented by the deciduous cypress (T. distichum), which extends from Florida to Texas. The two species of Sequoia, the " Wellingtonia " (S. gigantea) and the redwood (S. sempervirens), are confined to California. In the eastern forests the prevalence of Magnoliaceae and of Clethra and Rhododendron continues the alliance with eastern Asia. Florida derives a tropical element from the Antilles. Amongst palms the Corypheae are represented by Sabal and Thrinax, and there is a solitary Zamia amongst Cycads. The western dry areas have the old-world leguminous Astragalus and Prosopis (Mesquit), but are especially characterized by the north-ward extension of the new-world tropical Cactaceae, Mgmmillaria, Cereus and Opuntia, by succulent Amaryllideae such as Agave (of which the so-called " American aloe " is a type), and by arborescent Liliaceae (Yucca). Amongst palms Washingtonia, Brahea and Erythea (all Corypheae) replace the eastern genera. On the west coast Cupressus Lawsoniana replaces the northern Thuya gigantea, and a laurel (Umbellularia of isolated affinity) forms forests. California and Arizona have each a soecies of Platanus, a dying-out genus. Elsewhere it is only represented by P. occidentalis, the largest tree of the Atlantic forests from Maine to Oregon, and by P. orientalis in the eastern Mediterranean. Otherwise the Californian flora is entirely deficient in the characteristic features of that of eastern North America. Nor, with perhaps the interesting exception of C'astanopsis chrysophylla, the solitary representative in the New World of an east Asiatic genus, which ranges from Oregon to California, has it any affinity with the Chino-Japanese sub-region. Its closest connexion is with the South American Andine. II. THE TROPICAL REGION.—The permanence of continents and great oceans was first insisted upon by J. D. Dana, but, as already stated, has later received support on purely physical grounds. It precludes the explanation of any common features in the dissevered portions of the tropical area of vegetation by lateral communications, and throws back their origin to the remotest geological antiquity. In point of fact, resemblance is in the main con-fined to the higher groups, such as families and large genera; the smaller genera and species are entirely different. No genus or species of palm, for example, is common to the Old and New Worlds. The ancient broad-leaved Gymnosperm Gnetum has a few surviving species scattered through the tropics of both worlds, one reaching Polynesia. 1. African sub-region.—Western Arabia must be added to the African continent, which, with this exception and possibly a former European connexion in the far west, has had apparently from a very early period an almost insular character. Bentham remarks (Journ. Linn. Soc. xiii. 492) : " Here, more perhaps than in any other part of the globe, in Compositae as in so many other orders, we may fancy we see the scattered remains of ancient races dwindling down to their last representatives." It is remarkable that the characteristic features of the Miocene flora, which in other parts of the world have spread and developed southwards, are conspicuously absent from the African tropical flora. It has no Magnoliaceae, no maples, Pomaceae, or Vacciniaceae, no Rhododendron and no Abietineae. Perhaps even more striking is the absence of Cupuliferae; Quercus, in particular, which from Tertiary times has been a conspicuous northern type and in Malayan tropical conditions has developed others which are widely divergent. Palms are strikingly deficient: there are only three out of 79 genera of Areceae, and the Corypheae are entirely absent. But including the Mascarene Islands and Seychelles the Borasseae are exclusively African. Aroideae are poorly represented compared with either South groups of species and genera, have evidently diverged from stocks stands alone is that at least one-fifth and probably more of the species are common to both sides of the continent and presumably stretch right across it. An Indian element derived from the north-east is most marked on the eastern side: the Himalayan Gloriosa will suffice as an example, and of more tropical types Phoenix and Calamus amongst palms. The forest flora of Madagascar, though including an endemic family Chlaenaceae, is essentially tropical African, and the upland flora south temperate. 2. The Indo-Malayan sub-region includes the Indian and Malayan peninsulas, Cochin-China and southern China, the Malayan archipelago, and Philippines, with New Guinea and Polynesia, excluding the Sandwich Islands. Probably in point of number of species the preponderant family is Orchideae, though, as Hemsley remarks, they do not " give character to the scenery, or constitute the bulk of the vegetation." In Malaya and eastward the forests are rich in arborescent figs, laurels, myrtles, nutmegs, oaks and bamboos. Dipterocarpeae and Nepenthaceae only extend with a few outliers into the African sub-region. Screw pines have a closer connexion. Compositae are deficient. Amongst palms Areceae and Calameae are preponderant. Cycads are represented by Cycas itself, which in several species ranges from southern India to Polynesia. In India proper, with a dryer climate, grasses and Leguminosae take the lead in the number of species. But it has few distinctive botanical features. In the north-west it meets the Mediterraneo-Oriental and in the north-east the Chino-Japanese sub-regions, while south India and Ceylon have received a Malayan contribution. Bengal has no Cycas, oaks or nutmegs. Apart from the occurrence of Cycas, the Asiatic character of the Polynesian flora is illustrated by the distribution of Meliaceae. C. de Candolle finds that with one exception the species belong to genera represented in one or other of the Indian peninsulas. 3. The South American sub-region is perhaps richer in peculiar and distinctive types than either of the preceding. As in the Indo-Malayan sub-region, epiphytic orchids are probably most numerous in point of species, but the genera and even sub-tribes are far more restricted in their range than in the Old World; 4 sub-tribes with 74 genera of Vandeae are confined to South America, though varying in range of climate and altitude. Amongst arboreous families Leguminosae and Euphorbiaceae are prominent; Hevea belonging to the latter is widely distributed in various species in the Amazon basin, and yields Para and other kinds of rubber. Amongst Rubiaceae, Cinchoneae with some outliers in the Old World have their headquarters at cooler levels. In Brazil the myrtles are represented by " monkey-pots " (Lecythideae). Nearly related to myrtles are Melastomaceae which, poorly represented in the Old World, have attained here so prodigious a development in genera and .species, that Ball looks upon it as the seat of origin of the family. Amongst Ternstroemiaceae, the singular Marcgravieae are endemic. So also are the Vochysiaceae allied to the " milkworts." Cactaceae are widely spread and both northwards and southwards extend into temperate regions. Screw pines are replaced by the nearly allied Cyclanthaceae. The Amazon basin is the richest area in the world in palms, of which the Cocoineae are confined to South America, except the coco-nut, which has perhaps spread thence into Polynesia and eastward. The singular shrubby Amaryllids, Vellozieae, are common to tropical and South Africa, Madagascar and Brazil. Aroids, of which the tribes are not restricted in their distribution, have two large endemic genera, Philodendron and Anthurium. Amongst Cycads, Zamia is confined to the New World, and amongst Conifers, Araucaria, limited to the southern hemisphere, has scarcely less antiquity; Pinus reaches as far south as Cuba and Nicaragua. The flora of the Hawaiian Islands has complicated relations. Out of the 86o indigenous plants, 8o % are endemic, but Hillebrand finds that a large number are of American affinity. 1. The South African sub-region has a flora richer perhaps in number of species than any other; and these are often extremely local and restricted in area. It exhibits in a marked degree the density of species which, as already pointed out, is explicable by the arrest of further southern expansion. Hemsley remarks that " the northern genus Erica, which covers thousands of square miles in Europe with very few species, is represented by hundreds of species in a comparatively small area in South Africa." There is an interesting connexion with Europe through the so-called Iberian flora. Bentham (Pres. addr. Linn. Soc., 1869, p. 25) points out that " the west-European species of Erica, Genisteae, Lobelia, Gladiolus, &c., are some of them more nearly allied to corresponding Cape species than they are to each other; and many of the somewhat higher races, now unrepresented anywhere but in South Africa." This flora extends from Ireland to the Canaries and reappears on the highlands of Angola. On the eastern side the southern flora finds representatives in Abyssinia, including Protea, and on the mountains of equatorial Africa, Calodendron capense occurring on Kilimanjaro. This is the most northern representative of the Rutaceous Diosmeae, which are replaced in Australia by the Boronieae. The Proteaceous genus, Faurea, occurs in Angola and Madagascar. The characteristic genus Pelargonium has a few Mediterranean representatives, and one even occurs in Asia Minor. In all these cases it is a nice question whether we are tracing an ascending or descending stream. Darwin thought that the migration southwards would always be preponderant (Origin of Species, 5th ed., q.58). Other characteristic features of the flora are the abundance of Compositae, Asclepiadeae, and petaloid Monocotyledons generally, but especially Orchideae (terrestrial species predominating) and Iridaceae. There is a marked tendency towards a succulent habit. The nearly related Ficoideae replace the new-world Cactaceae, but the habit of the latter is simulated by fleshy Euphorbias and Asclepiads, just as that of Agave•is by the liliaceous Aloe. South Africa has only two palms (Phoenix and Hyphaene). In the Gnetaceous Welwitschia it possesses a vegetable type whose extraordinary peculiarities make it seem amongst contemporary vegetation much as some strange and extinct animal form would if suddenly endowed with life. Conifers are scantily represented by Callitris and Podocarpus, which is common to all three sub-regions; and Cycads by the endemic Encephalartos and Stangeria. 2. The Australian sub-region consists of Australia, Tasmania, New Caledonia and New Zealand, and, though partly lying within the tropic is most naturally treated as a whole. They are linked together by the presence of Proteaceae and of Epacrideae, which take the place of the nearly allied heaths in South Africa. The most dominant order in Australia is Leguminosae, including the acacias with leaf-like phyllodes (absent in New Zealand). Myrtaceae comes next with Eucalyptus, which forms three-fourths of the forests, and Melaleuca; both are absent from New Caledonia and New Zealand; a few species of the former extend to New Guinea and one of the latter to Malaya. Cupuliferae are absent except Fagus in Australia and New Zealand. The so-called " oaks " of Australia are Casuarina, which also occurs in New Caledonia, but is wanting in New Zealand. The giant rushes Xanthorrhoea and Kingia are peculiar to Australia. Palms are poorly represented in the sub-region and are of an Indo-Malayan type. Amongst Conifers, Podocar pus is found throughout, Agathis is common to Australia, New Zealand and New Caledonia; Araucaria the first and last. Of Cycads, Australia and New Caledonia have Cycas, and the former the endemic Macrozamia and Bowenia. The Australian land-surface must be of great antiquity, possibly Jurassic, and its isolation scarcely less ancient. In Lower Eocene times its flora appears to have been distinctly related to the existing one. Little confidence can, however, be placed in the identification of Proteaceous or, indeed, of any distinctively Australian plants in Tertiary deposits in the northern hemisphere. The Australian flora has extensions at high levels in the tropics; such exists on Kinabalu in Borneo under the equator. The Proteaceous genus Helicia reaches as far north as China, but whether it is starting or returning must as in other cases be left an open question. While the flora of New Caledonia is rich in species (3000), that of New Zealand is poor (1400). While so many conspicuous Australian elements are wanting in New Zealand, one-eighth of its flora belongs to South American genera. Especially noteworthy are the Andine Acaena, Gunnera, Fuchsia and Calceolaria. By the same path it has received a remarkable contribution from the North Temperate region; such familiar genera as Ranunculus, Epilobium and Veronica form more than q % of the flowering plants. And it is interesting to note that while the tropical forms of Quercus failed to reach Australia from Malaya, the temperate Fagus crept in by a back door. Three-quarters of the native species are endemic; they seem, however, to be quite unable to resist the invasion of new-comers, and already boo plants of foreign origin have succeeded in establishing themselves. 3 The Andine sub-region extends from Peru to the Argentine and follows roughly the watershed of the Amazon. In the New World, as already explained, the path of communication between the north-ern and southern hemispheres has always been more or less open, and the temperate flora of southern America does not exhibit the isolation characteristic of the southern region of the Old World. Taking, however, the Andean flora as typical, it contains a very marked endemic element; Ball finds that half the genera and four-fifths of the species are limited to it; on the other hand, that half the species of Gamopetalae belong to cosmopolitan genera such as Valeriana, Gentiana, Bartsia and Gnaphalium. The relation to the other sub-regions is slight. Ericeae are wholly absent, and it has but a single Epacrid in Fuegia. Proteaceae are more marked in Guevina and Embothrium. Of Restiaceae, a single Leptocarpus has been found in Chile. On the other hand, it is the headquarters of Mutisiaceae, represented in South Africa by such genera as Oldenburgia and Gerbera and by Trichocline in Australia. Tropaeolum takes the place of the nearly allied South African Pelargonium. There has been an interchange between it and the Mexico-American sub-region, but as usual the northern has been preponderant. Prosopis extendsto the Argentine; other characteristic genera are Oenothera, Godetia Collomia, Heliotropium and Eritrichium. In the ascending stream may be mentioned—Larrea, a small genus of Zygophylleae with headquarters in Paraguay and Chile, of which one species, L. mexicana, is the creosote plant of the Colorado desert, where it forms dense scrub; Acaena; the Loasaceae, of which Mentzelia reaches North America, Petunia and Lippia. Compositae compose a quarter of the Andean flora, which is a greater proportion than in any in the world. Baccharis, with some 250 species, ranges over the whole continent from the Straits of Magellan and, with seven species, to California. Melastomaceae, copiously represented in tropical America, are more feebly so in Peru and wholly wanting in Chile. A few Cactaceae extend to Chile. Of Cupuliferae, Quercus in three species only reaches Colombia, but Fagus, with only a single one in North America, is represented by several from Chile southwards and thence extends to New Zealand and Tasmania. The Magnoliaceous genus Drimys, with a single species in the new world, follows the same track. Bromeliaceae are represented by Rhodostachys and the temperate Puya. Palms as usual are few and not nearly related. Wettinia occurs in Peru, Trithrinax in Chile with the monotypic Jubaea, Juania, also monotypic, is confined to Juan Fernandez. Amongst Coniferae Podocarpus is common to this and preceding sub-regions; Libocedrus extends from California to New Zealand and New Caledonia; Fitzroya is found in Chile and Tasmania; and Araucaria in its most familiar species occurs in Chile. 4. The Antarctic-Alpine region is the complement of the Arctic-Alpine, but unlike the latter, its scattered distribution over numerous isolated points of land, remote from great continental areas, from which, during migrations like those attending the glacial period in the northern hemisphere, it could have been recruited, at once accounts for its limited number of species and their contracted range in the world. On the whole, it consists of local species of some widely distributed northern genera, such as Carex, Pea, Ranunculus, &c., with alpine types of strictly south temperate genera, characteristic of the separate localities. The monotypic Pringlea antiscorbutica, the " Kerguelen Island cabbage," has no near ally in the southern hemisphere, but is closely related to the northern Cochlearia. Such a summary of the salient facts in the, geographical distri.butiou of plants sufficiently indicates the tangled fabric of the earth's existing floral covering. Its complexity reflects the corresponding intricacy of geographical and geological evolution. If the surface of the globe had been symmetrically divided into sea and land, and these had been distributed in bands bounded by parallels of latitude, the character of vegetation would depend on temperature alone; and as regards its aggregate mass, we should find it attaining its maximum at the equator and sinking to its minimum at the poles. Under such circumstances the earth's vegetation would be very different from what it is, and the study of plant distribution would be a simple affair. It is true that the earth's physical geography presents certain broad features to which plants are adapted. But within these there is the greatest local diversity of moisture, elevation and isolation. Plants can only exist, as Darwin has said, where they must, not where they can. New Zealand was poorly stocked with a weak flora; the more robust and aggressive one of the north temperate region was ready at any moment to invade it, but was held back by physical barriers which human aid has alone enabled it to surpass. Palaeontological evidence conclusively proves that the surface of the earth has been successively occupied by vegetative forms of increasing complexity, rising from the simplest algae to the most highly organized flowering plant. We find the ultimate explanation of this in the facts that all organisms vary, and that their variations are inherited and, if useful, perpetuated. Structural complexity is brought about by the superposition of new variations on preceding ones. Continued existence implies perpetual adaptation to new conditions, and, as the adjustment becomes more refined, the corresponding structural organization becomes more elaborate. Inheritance preserves what exists, and this can only be modified and added to. Thus Asclepiadeae and Orchideae owe their extraordinary floral complexity to adaptation to insect fertilization. All organisms, then, are closely adapted to their surroundings. If these change, as we know they have changed, the organisms must change too, or perish. In some cases they survive by migration, but this is often prohibited by physical barriers, These, however, have often protected them from the competition of more vigorous invading races. Fagus, starting from the northern hemisphere, has more than held its own in Europe and Asia, but has all but died out in North America, finding conditions favourable for a fresh start in Australasia. The older types of Gymnosperms are inelastic and dying out. Even Pinus has found the task of crossing the tropics insuperable. The whole story points to a general distribution of flowering plants from the northern hemisphere southwards. It confirms the general belief on geological grounds that this was the seat of their development at the close of the Mesozoic era. It is certain that they originally existed under warmer conditions of climate than now obtain, and that progressive refrigeration has supplied a powerful impulse to migration. The tropics eventually became, what they are now, great areas of preservation. The Northern Temperate region was denuded of its floral wealth, of which it ,only retains a comparatively scanty wreck. High mountain levels supplied paths of communication for stocking the South Temperate region, the floras of which were enriched by adapted forms of tropical types. Such profound changes must necessarily have been accompanied by enormous elimination; the migrating hosts were perpetually thinned by falling out on the way. Further development was, however, not stopped, but in many cases stimulated by migration and settlement in new homes. The northern Quercus, arrested at the tropic in the new world, expanded in that of the old into new and striking races. And it cannot be doubted that the profusion of Melastomaceae in South America was not derived from elsewhere, but the result of local evolution. There is some evidence of a returning stream from the south, but as Hooker and A. de Candolle have pointed out, it is insignificant as compared with the outgoing one. Darwin attributes this to the fact that " the northern forms were the more powerful " (Origin of Species, 5th ed., p. 458). The result of migration is that races of widely different origin and habit have had to adapt themselves to similar conditions. This has brought about superficial resemblance in the floras of different countries. At first sight a South African Euphorbia might be mistaken for a South American Cactus, an Aloe for an Agave, a Senecio for ivy, or a New Zealand Veronica for a European Salicornia. A geographical botany based on such resemblances is only in reality a study of adaptations. The investigation of these may raise and solve interesting physiological problems, but throw no light on the facts and genetic relationship which a rational explanation of distribution requires. If we study a population and sort it into soldiers, sailors, ecclesiastics, lawyers and artisans, we may obtain facts of sociological value but learn nothing as to its racial origin and composition. In the attempt that has been made to map out the land surface of the earth, probable community of origin has been relied upon more than the possession of obvious characters. That sub-regions framed on this principle should show inter-relations and some degree of overlapping is only what might have been expected, and, in fact, confirms the validity of the principle adopted. It is interesting to observe that though deduced exclusively from the study of flowering plants, they are in substantial agreement with those now generally adopted by zoologists, and may therefore be presumed to be on the whole " natural."
End of Article: DISTRIBUTION OF
[back]
DISTRIBUTION (Lat. distribuere, to deal out)
[next]
DISTRIBUTION OF ENERGY IN INCOMPRESSIBLE FLUIDS

Additional information and Comments

There are no comments yet for this article.
» Add information or comments to this article.
Please link directly to this article:
Highlight the code below, right click and select "copy." Paste it into a website, email, or other HTML document.