Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/305

287 VEGETABLE.] 287 trees deciduous in winter, and its vast assemblage of her baceous types, Ranunculacew, Cruciferce, &c. These spread over Europe, northern and central Asia, and great part of North America. II. The Southern is broken up into numerous divergent floras. Their original connection is now traceable only in the common possession by two or more of them of large characteristic groups, such as Restiacece, Proteaccce, Dios- mea&amp;gt;, &c., the subordinate divisions of which have been locally specialized. To this belong the floras of extra- tropical South America, South Africa, and Australia with New Zealand, to which must probably be added an area borrowed from the northern hemisphere in Mexico and California. III. The Tropical is characterized by the predominance of mostly evergreen arborescent Polypetalce (Anonacece, Meliacece, Leguininosce,-&v.), and gigantic Monocotyledons, of which Palms, Scitaminece, and Bambusece amongst grasses are especially striking. I. THE NORTHERN FLORA. This has been long divided into that of Old and New World by the severance of North America from Northern Asia, and by the barrier to an interchange of vegetation in the upheaval of the Rocky Mountain range. Nevertheless its marked con tinuity (with only a gradual east and west change in the arctic regions, but an increased divergency southwards) requires it to be treated as a whole. The Old and New World divisions of this flora, which, no doubt, began to diverge from the mere influence of distance, have nowhad that divergence immensely increased by isolation. According to Lesquereux, 1 the essential types of the present arborescent flora of North America are indicated in the Cretaceous rocks of that country, and become more distinct and numerous in the Tertiary; and he believes that the origin of the existing American flora is American. The analogy between the Miocene flora of Central Europe and the present North American flora is unquestioned, and is greater than between the same fossil flora and that now existing in Europe. Lesquereux s Conclusion is that tho American element in the vegetation of Miocene Europe was derivative, and this is one of many illustrations of the curious observation of Asa Gray that plants have in general a greater tendency to migrate from east to west than from west to east. This Miocene flora was, however, gradually driven back again, and it is only as we travel from Europe to the East that we gradually find its traces getting stronger and stronger. Thus, as Oliver 2 has pointed out, in passing from the Mediterranean to the Levant, the Caucasus, and Persia, we meet with living representatives of the Miocene genera Chamcerops, Platanus, Llquidambar, Pterocarya, Juylans, &c. Along the Himalayas and through China we trace other Miocene genera, Japan forming part of the same botanical region as Eastern Asia. Among the remarkable existing North American types which may be mentioned as reappearing in the Himalayas and Japan are Aralia quinguefolia, Phryma leptostachya. and Trillium erectum. One of the most interesting additional facts which has recently come to light is the occurrence of a species of tulip tree (Liriodendroii) in Central China, which genus, though a member of the European Miocene flora, has in recent times bsen regarded exclusively characteristic of America. 3 With respect to other American genera which are not necessarily part of the Miocene flora, the same general principle holds good. Bentham remarks, that while some, like Astragalus, have multiplied largely in both continents, &quot; other genera, like JKtqxtiorium, Aster, Phlox, Solanum, &c., very nume- 1 Geological Survey of Montana, 1871, p. 314. 2 Nat. Hist. Rev. 18G2. 1 Moors, Journ. of But. 1875, p. 225. rou sly represented in America, have transmitted or produced a smaller number in Eastern Asia, gradually diminishing westward till they disappear altogether or attain Western Europe in single species but little altered from American ones.&quot; 4 The Europeo-Asiatic genera, on the other hand, such as Cruciferce, Umbettiferce, &c., which are so dominant a feature in the existing Old World Northern flora, appear &quot; to have left but few representatives in America, and those much more modified than the American races left in Asia.&quot; Besides the internal migrations of the various constitu ents of the great Northern flora, its boundaries have been changed longitudinally under the influence of secular varia tions of climate alluded to above. The nature of these cannot be better summed up than in the words of Bentham : 5 &quot; Where the chief portion of this greut northern flora originated, and whether it may be best termed Scandinavian, or North Asiatic, or Caucasian, is a question for the determining of which we have little or no data ; but, as observed by Hooker, it is probably one of the most ancient and widest spread, having at different epochs travelled over a great part of the globe. Shown by the researches of Lesquereux, as well as by the recent ones of Heer and others, to have extended far north during the warmer preglacial times, it must have been slowly driven southwards as the glacial epoch came on, and either then, or at some one or more other periods, have been for a time continuous, in two lines at least, into the southern hemi sphere ; for it has left traces still discernible, especially in its herbaceous and mountain forms, in the mountains of tropical Asia, down at least to the Indian peninsula, and westward to the Abys sinian and Cameroons mountains of Africa, and, again, down the Andes to the extreme south of America, where it is still luxuriant, and in a less degree in New Zealand, Tasmania, and Victoria. In all these migrations, whilst retaining a general identity, the flora must have undergone continual changes, losing species or other races of limited areas and propagation as their habitations became unfit for them, and gradually forming new ones when favoured by long-continued isolation or other requisite conditions.&quot; The Northern flora has further undergone a specialization into three secondary floras, due to the combined influence of physical and genetic causes. 1. The Arctic-alpine flora (&quot; consisting chiefly of plants of small stature, slow growth, and limited means of dis persion, compensated by long lives and great powers of endurance &quot;) is perhaps the most interesting of the three subdivisions, both because in its arctic aspect it reduces the divergence of the Old and New World divisions of the Northern flora to a minimum, and more especially on account of the great interest which attaches to the problem of its scattered alpine outliers. With regard to the first point, Hooker found that estimating the whole Arctic flora at 762 species, Arctic East America possessed 379, of which 269 were common to Scandinavia. Of the whole flora 616 species are found in Arctic Europe, and of these 586 are Scandinavian, and this leads Hooker to the striking observation that &quot; the Scandinavian flora is present in every latitude of the globe, and is the only one that is so.&quot; 6 Christ objects to Hooker s giving the title of Scandinavian to the Arctic flora, but we must agree with Bentham 7 that Scandi navia &quot;would, according to older rules, have been regarded as the centre of creation for the arctic lands, and may now be termed the chief centre of preservation within the arctic circle, owing perhaps partly to its more broken conforma tion, and partly to that warmer climate which, while it now admits species which Christ objects to being included in the Arctic flora, was during the glacial period a means of preservation of some colder species which were everywhere else e*xpelled or destroyed.&quot; Just as at present the Arctic is more homogeneous than 4 Presidential address, 1869, p. 18 ; see also Beutham in Journ. Linn. Soc. Bot., xiii. p. 500. 5 Presidential address, 1869, p. 19. 6 Hooker, on the &quot;Distribution of Arctic Plants,&quot; Tram. Linn. Soc., vol. xxiii. p. 253. 7 Presidential address, 1869, p. 21.