Introduction. 3 



converse hypothesis of a warm climate continuous from 

 Preglacial times to account for the Iberian plants in the 

 west of Ireland and in Cornwall. Either might be true, 

 but scarcely both ; for the Irish and Cornish plants are not 

 such as could survive a colder climate like that postulated 

 by Forbes to explain the migration of the Arctic species. 

 We have obtained direct evidence, since Forbes wrote, 

 that all Ireland was at one time strongly glaciated, and 

 also that Arctic plants once occupied the lowlands of 

 Devonshire. 



This problem of the origin of our flora is one which can 

 be solved, I think, by the historical method, and that 

 seems to be the proper mode of attacking it. No doubt 

 the imperfection of the geological record is so great as to 

 make the task an exceedingly difficult one ; for nowhere 

 have we yet discovered a continuous sequence of deposits, 

 all fossiliferous, such as would give a connected history of 

 our recent animals and plants from their first appearance in 

 Britain to the present day. The exact order of succession 

 of the deposits, of the physical changes, of the climatic 

 alternations, and of the waves of migration, is still uncer- 

 tain ; though a definite historical record is gradually being 

 built up by the comparison and correlation of numerous 

 overlapping chronicles, each recording at most some three 

 or four of the subordinate stages or periods. This work of 

 correlation, as already mentioned, has been greatly 

 facilitated by a detailed examination of extensive areas, 

 and a close study of the geology of the more recent deposits. 

 In this way I have been enabled to trace the connexion 

 between the strata, and often to speak with confidence as to 

 the date of groups of fossils which otherwise would have had 

 to remain as isolated finds. My own researches have been 

 largely aided and supplemented by the examination of 

 material obtained from friends working in districts which I 



