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appear, placed in certain regions, date more or less from single pairs of species 

 dispersed from primitive centres according to their capacity to endure change 

 of climate, and the absence of physical obstacles to emigration. 



In the case of plants, however difficult the task to fix on the true 

 aborigines — whether such was a single individual, a pair, or an indefinite num- 

 ber — yet the very forms which we regard now as typical may be only one of 

 the many variaiions produced by time and the variable and ever- varying con- 

 dition of climate. In the next place, we cannot fix the date of such agencies 

 at work ; we may, however, reasonably infer that the modifications must 

 depend on the special vitality of each species, and its power to resist and over- 

 come obstacles, and so become cosmopolite in its range. Taking De Candolle's 

 "Prodromus" as our guide, no flowering plant is an absolute cosmopolite. 



Under corresponding parallels of latitude, at the same elevation, and 

 with kindred soils, we find distinct genera, and even a genus confined solely 

 to one side of our planet. Such is the Erica, confined as it is to the narrow 

 zone extending from the North of Europe to the Cape of Good Hope. 



There is no Rose in South America, although the family Rosaciaj is 

 represented there. 



Coming nearer borne we have many plants stopping short of our higher 

 altitudes, yet ascending elsewhere to much more boreal habits, — the Juniper 

 and the Pyrus aucuparia for example. 



In the Grimsel the Pinus Sylvestris has been found flourishing up to 

 6,330 feet above the level of the sea, Betula alba to 6,916 feet, and beyond 

 them the Pinus Cembra to 7,348 feet, so that here the usual limit of deci- 

 duous trees is suspended, and the Birch lies between two belts of Conifers 

 (Humboldt's " Cosmos "). 



Mr. Darwin, on crossing the Andes of Chili, was struck with the marked 

 differences of the vegetation in the valleys each side, the climate and the soil 

 the same, and the difference of longitude veiy trifling (Darwin's Travels). 



Despite homozoic belts, and isothermal lines, there still remains this 

 enigmatical fact, this inexplicable caprice, unless we concede that the genera 

 in past times have had a much wider range than now, however distinct and ex- 

 clusive the endermic vegetation of any given district may appear. 



The present distribution of sea and land is exceedingly irregular, and 

 what is the geologic record ? but, that many other equally abnormal distri- 

 butions have occurred, and with such, changes of climate, causing the dis- 

 appearance of certain forms and the occuirence of others in places we do not 

 now expect to find them. 



To give one instance only out of many. At the Miocene period, or, 

 more strictly, at its dawn, a temperate clime prevailed within the Arctic circle. 

 Poplais, Planes, and Lime trees grew within twelve degrees of the Pole, and so 

 have beds of the same fossil plants been found in North Greenland, wheie now 

 an enormous ice cap covers the whole country, leaving only a narrow slip of 

 land free from ice in summer, and where but a few dwarf willows can exist. 



