468 GEOGRAPPIICAL DISTRIBUTION OF PLANTS. 



a rather deeper sea. If sands are superimposed upon the London clay, 

 they cannot be expected to bring back again the fauna of Thanet 

 sands, any more than a clay newer than the London clay could be 

 expected to repeat the London clay fauna, for the reasons which have 

 been shadowed forth in stating the laws of upheaval of land. The 

 palaeontologist may suspect physical changes which the geologist has 

 no proof of, but more frequently his task is to demonstrate the events 

 which stratigraphical conditions exemplify ; and in any case he must 

 work out the physical history of the deposits before co-ordinating the 

 faunas. 



Distribution of Plants. If plants are to be used as an instru- 

 ment for research by which to investigate physical mutations and 

 revolutions in the distribution of land and water, it is impossible to 

 neglect their present geographical distribution. For important as are 

 temperature, moisture, and soil in governing the diffusion of plant life, 

 when it has reached a region of land, it is to geological changes upon 

 the earth's surface that we look for the only agent which was capable 

 of severing floras by the upheaval of mountains and deserts, or by the 

 depression of seas ; while the upheaval of land alone blends floras 

 which seas previously divided. The changed distribution of plant 

 life which the strata make known, lays before us some of the stages 

 by which the existing floras have been distributed, denned, and 

 evolved. 



There are a few water plants like Potamogeton and Ceratophyllum 

 demersum which range almost from pole to pole. Other species, like 

 the Pteris aquilina, range from Lapland and the upper limits of culti- 

 vation to the banks of the Amazon. Few genera have a world- wide 

 range; but among the most widely distributed are Lotus, Rubus, 

 Plantago, Typha, Oxalis, Nasturtium, Gnaphalium, and Senecio. But 

 for the most part the distribution of plants is limited by the distribu- 

 tion of land, and denned by climate, so that zones, such as are named 

 tropical, temperate, and arctic, may be characterised by floras. 



Bentham l suggests that the great botanical and zoological regions 

 probably coincide in general disposition, but that the plant regions are 

 older than the animal regions. There is first a northern type, secondly 

 a tropical type, thirdly a southern type. The northern type is espe- 

 cially marked by its needle-leaved conifers, Arnentacese, Eanun- 

 culacese, Cruciferae, and Trifolieas. It spreads over Europe, Northern 

 and Central Asia, and a part of North America, but has long been 

 divided by the Atlantic and Pacific Oceans. Asia has preserved 

 American types in Japan, Manchuria, and the Himalayas, occasionally 

 with identical species, but chiefly with representative species. 

 Astragalus is equally common in both continents. But other genera, 

 like Aster, Flox, Solanum, Eupatorium, are more numerous in 

 America than in Eastern Asia, dwindling towards Europe. On the 

 other hand, the European and Asiatic genera of CruciferaB, Caryo- 

 phyllea3, Lotese, and UmbelliferaB, have but few representatives in 

 America. In the pre-glacial period the northern flora extended far 

 1 See all Bentham's addresses to the Linnaean Society, especially 1869. 



