BOTANICAL GEOGRAPHY. 4.21 



different geological epochs ; but if, as I believe to be the case, climatal con- 

 ditions sufficient to account for the above distribution of British plants were 

 present, the error would not lie in the method, but its application, which has 

 led Forbes to the following results. According to his view, the districts of 

 vegetation distinguished above correspond to as many geological eras, so that 

 the flora of the west of Ireland would be the oldest, that of the mountains 

 the fourth, and that related to Germany the youngest. The first mentioned 

 descend from a time when a chain of mountains, running across the Atlantic 

 Ocean, connected Ireland with Spain ; this would explain its difference from 

 the vegetation of the mountains, although it would still correspond with the 

 mountain character. Moreover, in the second and third periods the English 

 Channel was closed, first towards the west, then towards the east also, by the 

 connexion of land, and thus the distribution of French plants in England was 

 occasioned. Forbes explains the alpine flora of the mountains by means of 

 Agassiz's glacial period ; the mountain summits of Britain were then low 

 islands, extending to Norway, and were clothed with an arctic vegetation, 

 which, after the gradual upheaval and consequent change of climate, became 

 limited to the summits of the newly-formed and still existing mountains. 

 Lastly, the bed of the North Sea itself was upheaved, and extensive plains 

 laid dry between England and Germany, upon which the elk and other extinct 

 quadrupeds lived, and over which the plants of Germany migrated ; until at 

 last the sea, in consequence of fresh depression, flowed back, after the im- 

 portant object of transplanting Roses and RuU beyond the ocean had been 

 accomplished. Further than this, hypothetical views could not easily be 

 carried, and I have translated them almost entirely here, only because Forbes 

 appears desirous by this paper of opening a new path in botanical geography, 

 for this first lecture has since been followed by others. The criticism of his 

 undertaking lies simply in the denial of one of the first statements with 

 which he commences : actual natural forces, the sea, rivers, currents of air, 

 by which seeds are diffused, or animals, and even man, are, in the majority 

 of cases, insufficient means for effecting the migration of plants across the 

 British seas. I maintain that these forces are quite sufficient, provided the 

 imported seeds meet with a suitable climate and proper soil. Those western- 

 Europe plants, which, being produced through the agency of the coast - 

 climate of the Atlantic, and, according to the degree of this dependence, 

 becoming distributed sometimes to a greater, sometimes a less distance 

 within the continent, the author refers in the latter case to Spain, in the 

 former to France. are not met with equally on the coast-line of the con- 

 tinent, but are often absent from wide tracts, the soil of which is not 

 favorable to their growth ; when e. g, we do not find Erica cinerea anywhere 

 from the Rhine to the Fjord of Bergen, who would, in this case, suppose 

 that former connexions of land had disappeared, when for the most part the 

 connexion still exists, without contributing to the distribution of this shrub ? 



