420 BOTANICAL GEOGRAPHY. 



gate of heterogeneous laws belonging to different sciences ; for in this light 

 only does it present a definite and independent method of investigation, a 

 progressive course of development.. Commencing with observations upon 

 the geographical area of each individual species of plant, the object of 

 botanical geography is first to determine what limits to this distribution have 

 been placed by the composition of the soil or the subdivision of the continent. 

 It then points out the climatal sphere of the species, and if it ascertains, 

 after this twofold limitation, that the natural area is more contracted than is 

 accounted for, the geological problem presents itself what has not been the 

 result of soil and climate must depend upon historical causes, the history of 

 the earth. If the same soil and the same climate have produced only similar 

 but not identical forms, this refers us to a creative act of a different kind, 

 therefore to a geological epoch. 



In connexion with this combination of geological and botanico-geographical 

 investigation, Forbes has made an attempt of a different kind, viz. the appli- 

 cation of the distribution of plants to geological deductions (Report of the 

 Meeting of the British Association, held at Cambridge, Ann. Nat. History, 

 xvi, p. 126). On comparing the specific centres of the endemic plants of 

 Great Britain, i. e. the centres of their geographical areas, it is evident that 

 the flora of the greater part of the surface of the country belongs to that of 

 Germany. The specific centres of the few species peculiar to the British 

 Isles occur in the same region. In addition to this principal area, four 

 smaller districts of vegetation may be distinguished according to the same 

 law : 1. The mountainous districts of the west of Ireland contain a number 

 of plants in common with the north-west of Spain and the Pyrenees. 2. The 

 south country, Devonshire, Cornwall, and the Channel Islands, in common 

 with the west of Trance. 3. The south-east of England, especially its 

 chalky districts, with the north of France. 4. The mountains of Wales, the 

 north of England and Scotland, with the plains of Norway. Forbes does 

 not .consider this connexion as explicable by soil and climate, and therefore 

 seeks for geological causes, in conformity with the above law. He believes 

 that these are to be found in a former connexion of Great Britain with the 

 continent, probably existing in earlier geological periods, especially during 

 the tertiary period : not that this connexion, made use of for his explanation, 

 has been geologically determined ; but he endeavours to support his geological 

 hypotheses by these botanico-geographical relations. Following up this 

 design, which is certainly not free from objection, Forbes then not only 

 maintains generally these connexions of land, but by supposing former eleva- 

 tions and subsidence of the soil, arrives at certain views regarding the series 

 of changes which have occurred ; in fact, he distinguishes the floras according 

 to the periods at which they were formed. I should have no hesitation my- 

 self in granting, that when two different floras really belong to the same soil 

 and climate, by far the simplest hypothesis is to attribute their origin to 



