rainfall, that we should look for the ancestors of many forms of vegetation which have stamped their character on the vegetation of the continent. In addition, the mountains of Venezuela and Guiana, representing the southern part of the Antillean land extension may be considered also as a center from which the tropic flora has spread. This region during lower and upper Cretaceous periods was severed from eastern Brazil and during that time an endemic tropic flora was in process of development. This flora is characterized by the great richness in Araceae and Orchidaceae. While separated from the south and also with several distinct orographic systems small areas existed which developed peculiar endemic forms for which this region is noted. No part of America has been more frequently visited by European plant importers in search of aroids, palms, orchids and bromeliads than this. Later northern South America was connected with eastern Brazil and it was then that the plants mentioned by BALL migrated north to Guiana, where they form an important element, and subsequently into Central America and the West Indies. The tropic element of the North American flora represents three distinct elements of development, viz., the subandine region, which according to ENGLER furnished a large contingent of species, the Antillean continent including Venezuela and the eastern Brazilian district, which subsequent to the Cretaceous period furnished a considerable number of migrant forms which found their way northward, while the higher Andes contributed many types to the higher Mexican mountains. We are not in a position to clearly separate from each other the plants derived from all these areas, but we have proceeded far enough to discover that the Mexican, Central American, West Indian and Brazilian tropic floras are all connected by identic species of plants. If diversity has occurred, it has been through the isolation of areas of great physiographic difference permitting the differentiation of new types of plant life from old forms. This separation of land masses into groups by the encroachment of the sea has also separated widely distributed forms which remain as relicts in circumscribed areas, or it has led to the extinction of many forms once widely prevalent. West Indian Flora. GRISEBACH) early compared statistically the flora of the different West Indian islands under English control and the facts discovered by him bear out the statement just made, that although, the Central American, northern South American and West Indian floras are genetically related historically, yet sufficient time has elapsed since the disruption of the Antillean continent to bring about a rich endemism in the several regions. Our knowledge of the West Indian flora has vastly increased since 1864, but we still lack sufficient data on which to make a satisfactory and complete statistic comparison of this flora with others adjacent to it. We have brought out some of the salient features in a preceding chapter. It only remains to say in this connection, that the table in GRISEBACH's great work is helpful in 1) GRISEBACH: Geographische Verbreitung der Pflanzen Westindiens. elucidating the fact that there is a greater endemism in each of the Greater Antillean islands than in those of the Lesser Antilles, for the simple reason that the large islands are continental in aspect, have sedimentary rocks and represent part of the original Antillean landmass, which was covered by an Antillean flora of great richness, while the islands of the Lesser Antilles are volcanic and of comparatively recent creation, representing a line of weakness of the earth's crust through which volcanic material has been extruded along the eastern edge of the lost Atlantis (Antillean continent), when it sank out of sight with the formation of the American Mediterranean, the Caribbean Sea. The flora of these islands, therefore, is a recently derived one and represents one that has not had sufficient time for the differentiation of new endemic types, hence the discrepancy in the number of endemic types in the two groups of islands which the table of GRISEBACH sets forth. Presumably the appearance of the Caribbean Sea by the depression or sinking down of the former land surfaces of the Antillean continent to great depths below the surface separated the Antillean flora into three parts, viz., the West Indian, the Central American and the northern South American. The West Indian flora was at one time perhaps remarkably uniform for during the close of the Tertiary period, we have evidences that the West Indian land areas ▾ were much more extensive than now, and the greater Antilles were once continuous. With the depression of the islands in subsequent periods, the West Indian landmass was broken up into physiographically distinct regions and the present differentiation of the flora began at this time. So that for the several larger islands of the Greater Antilles, we have a diversification of the flora which has been statistically stated in a foregoing chapter of this book. The Bahamas are very recent geologically speaking and their elevation. above the sea has been placed not carlier than the late Tertiary, so that excellent opportunities are afforded in this group to study plant migration and evolution. The flora has been remarked in a former page is of southern derivation, a large number of the known indigenous species being common to the nearby and older islands of Cuba and Haiti, while many other species are closely related to plants from these islands. The chief agents in the introduction and distribution of the plant population are according to BRITTON) migratory birds, supplemented by winds and ocean currents. Notwithstanding the geologically short period that the Bahama islands have been above the sea, they have witnessed the evolution by mutation, or otherwise, of numerous species, there being many endemic species known and many more which will be made known as the result of recent explorations. We have then to deal before the glacial period with seven distinct regions with types of vegetation developed during the Cretaceous and Tertiary periods in situ, and which to some extent migrated from area to area, as the relative position of the land 1) BRITTON, N. L.: A botanical Cruise in the Bahamas. Science new ser. XXI: 628. April 21, and waters were altered. These seven types are: the circumpolar arctic flora, the north temperate continental forest flora, the Pacific forest vegetation, the grass land vegetation, the Mexican highland flora, the South American and Antillean continental tropic floras. It was by the combination and recombination of these several types of vegetation, the sorting and geographic re-arrangement of species in geologic periods subsequent to the Cretaceous period, especially during the glacial period, that the distribution of the plants of the present North American flora is directly traceable. Chapter IV. Affinities of North American Flora. 1. Arctic America. In considering the affinities of the flora of arctic North America, it is necessary to present a few facts and statistics about the arctic flora of the world in general. The arctic flora occupies a circumpolar area north of the arctic circle. There is no abrupt break in the vegetation anywhere along this belt, except at Baffin Bay, where a sudden change from an almost purely European flora in Greenland on its east coast, to one with a large admixture of American plants on its west. Regarded as a whole, the arctic flora is decidedly Scandinavian, for arctic Scandinavia, or Lapland, though a very small tract of land, contains by far the richest arctic flora, amounting to three fourths of the whole; more over, according to HOOKER, upwards of three-fifths of the species, and almost all of the genera of arctic Asia and America are likewise Lapponian, leaving far too small a percentage of other forms to admit of the arctic Asiatic and American floras being ranked as anything more than subdivisions of one general arctic flora. The American district, omitting Greenland, is separable into two districts, the eastern American and the western American, separated from each other by the estuary of the Mackenzie River. The North-American districts. Arctic western America extends from Cape Prince of Wales on the east shore of Bering Strait to the estuary of the Mackenzie River, and as a whole, it differs from the flora of the district to the east by the far greater number of both of European and Asiatic species by containing various Altaic and Siberian plants, which do not reach so high a latitude in more western meridians and by some temperate plants peculiar to western America. The number of phanerogamic plants found in arctic western America is approximately 364 species. Of these 364 species almost all but the littoral and purely arctic species are found in west temperate North America, or in the Rocky Mountains, 26 in the Andes of the tropic, or subtropic America and 37 in temperate or antarctic South America. Comparing this flora with that of temperate and arctic Asia, no less than 320 species are found on the north-western shores and islands of that continent, or in Siberia, many extending to the Altai and Himalaya Mountains. A comparison with eastern arctic America shows that 281 are common to it, and that 38 are found in temperate, but not arctic eastern America 1). The flora of arctic east America differs from that of the western part of the continent, in possessing more east American species. The western boundary is an artificial one, but the eastern is natural both botanically and geographically, for Baffin Bay and Davis Strait have very deep water and different floras on their opposite shores. The portion of this province richest in plants is the tract between the Coppermine and Mackenzie rivers. East of this the number of plants rapidly diminishes and also to the northward. The flora of arctic east America consists according to HOOKER of 379 species. Of these 379 species, 323 inhabit temperate North America, east of the Rocky mountains; 35, the Cordilleras; and 49, temperate antarctic South America. Comparing this flora with that of Europe, it is found that 239 species are common to the arctic regions of both continents, while but little more than one third of the arctic European species are arctic east American. Of 105 non-European species in arctic east America, 32 are Asiatic; leaving 73 species confined to America. Douglasia arctica and Pleuropogon Sabinii are the only plants absolutely peculiar to arctic east America. Compared with Greenland, the flora of arctic east America is rich including many species not found in Greenland. The following are found on the arctic islands, and many of them on the west coast of Baffin Bay, but not in west Greenland. 1) HOOKER, J. D.: Outlines of the Distribution of arctic Plants. Transaction Linnean Society of London XXIII: 251-348. 1860. No fewer than 184 of the 379 arctic east American species (fully half) are absent in west Greenland, whilst only 105 (much less than one-third) are absent in Europe. Of the 379 arctic east American species only 56 are not found in temperate east America, of which two are absolutely confined to this area; two others, Parrya arenicola and Festuca Richardsoni, to arctic east and west America; 25 are found in temperate west America, and about 20 are Rocky Mountain species, and not found elsewhere in temperate America. Algae. The strong endemism of the purely arctic marine flora points. to it as no immigrant flora, but one that possesses its center of development in the Arctic Sea. Other circumstances lead cogently to the same conclusion, indicating at the same time that the present purely glacial marine flora must have been formerly more widely spread towards the south than it is now. This results from a comparison of the flora of the Arctic Sea with that of the northern Atlantic and the northern Pacific. The Arctic Sea possesses 184 species in common with the North Atlantic and only 11 of these species are exclusively American, for by far the greater number occur on the Atlantic. coasts of Europe according to KJELLMAN). The present flora in the northern part of the Pacific differs so essentially in composition from that of the northern Atlantic, that is to say, it contains many species that are so sharply distinguished from those of the Atlantic, even belonging to quite different types, that in order to account in any way for this fact, one is necessarily obliged to assume that these two divisions of the ocean appertain to different areas of development within which different forms have continued to be evolved during a very long time.. (Confer Phyllospadix: Fig. 11, p. 314.) However, on the other hand, it is a well known fact that the northern Atlantic has no inconsiderable number of species in common with the northern Pacific. Comparison of North Temperate and Arctic Liverworts. In the north temperate and arctic zones, according to UNDERWOOD) (1892), there are known about 575 species of liverworts, Musci hepaticae. Of these 375 belong to the flora of Europe, 300 to that of America, and perhaps 150 to that of Asia. Of these, we may take, as representing the boreal and sub-boreal portions, 173 species for northern Europe, 163 for northern America, and 98 species for northern Asia. Of the 214 boreal and sub-boreal species, eighty per cent are European, seventy-six per cent are American, and forty-six per cent are Asiatic. While the larger part of the species of Europe and America have been brought to light, it is quite likely that the smaller number known from the more extensive Asiatic continent is due to the limited exploration of that region. Of the 163 American species, 129 or seventy-eight per cent are of the European flora; 69 are also Asiatic, while 32, or twenty per cent, are endemic. 1) KJELLMAN, F. R.: The Algae of the Arctic Sea. 50. 1883. 2 UNDERWOOD, LUCIEN M.: A preliminary Comparison of the hepatic Flora of boreal and sub-boreal Regions, Botanical Gazette XVII: 305. October 1892. |