and animal life must have their greatest effect, here being the greatest mixture of air with the water; the greatest motion of the water, to give full effect to that mixture; the greatest amount of stimulating light, and the greatest change of daily temperature; we shall readily conceive that near the surface the forms of life should be both more varied and more abundant; while, on the other hand, in the deeper and calmer water, less light, less motion, less air and less change of air, should correspond to fewer and less varied inhabitants. Only a few of the Mollusca are, like the Argonaut of the Poet, who tilts along the Atlantic waves— But if a breath of danger sound, With sails quick-furled she dives profound, In coral grots defies the foe, That never broke, in heaviest wrath, The sabbath of the deeps below. In a Diagram, Fig. 2, we may represent the limits of life above and below the level of the sea by setting off on the vertical line a scale of heights and depths, and drawing at right angles to this, for each selected zone of height and depth, lines representing the ratios of abundance of life. Thus shall we have, in a general form, an expression of the apparent dependence of life on elevation above and depression below the general level of the surface of the sea. The real dependence on the land is R. L. C ascertained by the temperatures placed opposite the several elevations. PROVINCES OF LIFE. We have been speaking of the climatal distribution of life-forms in general, and of the larger groups and families. The distribution of genera and of species seems to obey the same general laws of co-ordination to climate, elevation, and depth; but it also often suggests the dependence on existing purely local conditions, on ancient geological revolutions, or even on locality considered alone and without regard to conditions. The great group of Ferns occurs in nearly all latitudes; Arborescent Ferns, excluded from cold and fluctuating climates, extend further to the south than to the north of the equator. Thus, Aspidium is arborescent on the shores of Auckland and Campbell, as far as 52 S. Lat.; Alsophila and Cybotium in Australia and Van Diemen; Cyathea and Dicksonia are conspicuous in the vegetation of New Zealand. An example of the limitation of a race to terrestrial conditions is afforded by the Gorilla; that monstrous anthropoid animal of the eastern coast of Africa, whose residence seems limited by the forests which supply it with food. On the contrary, it is to ancient geological revo lutions that we must ascribe the detached aspect of the distribution of many plants and animals ; these great changes having divided what was once a continuous area, and interrupted what was once a free communication between the regions in which the species occur. Thus we may comprehend the occurrence of Scandinavian plants in the mountains of Scotland, and the north of England; as Myosotis alpestris, on Micklefell 2400 ft. high, in Yorkshire; Cornus Suecica on the eastern moorland, and Trientalis Europæa, in the western part of the same county. We must suppose land once continuous between Scotland and Norway, and a climate more severe than the present. Under such circumstances the plants might spread southward; and afterwards, when the countries were divided and the climate became milder, they might remain in a few and especially cold or elevated situations1. In the same way we may attempt to explain the remarkable fact that not only a very considerable number of European plants is found in the Himalaya mountains, but also many of the accompanying birds. The late Dr Royle long since called attention to this remarkable fact, and Dr Hooker, recently, has stated that no less than 222 species of British plants extend to India, and oblige us to look to a common 1 E. Forbes, in Memoirs of Geological Survey, Vol. 1. origin for the species found in both these regions, and to seek for causes no longer in operation for their distribution over so extended an area1. It is remarkable that these 222 species include no less than 154 genera. Among them are the plants which flower in fields and meadows, in woods and wastes, in marshes and in water-trees and herbs, climbers and parasites, reproduce before us the familiar aspect of European vegetation. The water-plants include Ranunculus aquatilis, Nymphæa alba, Myriophyllum verticillatum, Hippuris vulgaris, Alisma plantago, Sagittaria sagittifolia, Butomus umbellatus, Acorus calamus, and several species of Potamogeton. East of Kumaoon, the European admixture of plants gives place to a Chinese and Malayan flora. West of the Himalaya, mountain-plants of the European type occur at several points, ideally connecting this range with the coasts of the Levant and the Black Sea, and really indicative of the anciently connected flora and the continuous means of communication. . Another inference from such facts is of equal importance. The species of plants thus shewn to be identical in the regions of the Himalaya and the Islands of the West, have retained their characters during all the time of the existence of each race; these characters have survived the chances of long 1 Flora Indica. Introductory Essay, 108. migration, have traversed large tracts without hybridism, and have been subject to various physical influences without suffering any sensible change. Ranunculus aquatilis in India and Britain every where retains the little nectary at the base of each petal, to mark the genus, still spreads its white blossoms on the water, and extends its filmy leaves beneath. Still Ranunculus lingua distinguishes itself by its leaves, and Ranunculus arvensis by its prickly seeds. Instances of purely local species of plants are very common; sometimes defined by impassable boundaries-as broad oceans or crested mountains ;but not unfrequently contracted to small areas from a once wider distribution, or confined to such areas by the successful rivalry of other and more prosperous races. Thus the huge cypresses in the Sierra Nevada of Upper California, known to us as Wellingtonia1, are now confined to a small district, and indeed rear their prodigious heads 400 ft. high, mostly in one valley, where they have lived for a thousand years and more near the head-waters of the Stanislaus and St Antonio rivers, lat. 38° N., long. 120° 10' W.2 So in the valley of the Cherwell, in Oxfordshire, the Fritillaria adorns but a few meadows, 1 Sequoia Wellingtoniana is perhaps the right appellation. |