Introduction. 7 



and at present few of the numerous fossil species occurring 

 in our Pleistocene deposits have been determined. Fresh- 

 water mollusca, freshwater fish, and amphibia seem to obey 

 the same laws of geographical distribution as aquatic 

 plants : the species are usually of wide range, provided the 

 barriers are not excessively broad or high, and the 

 climatic conditions are suitable. 



The geological sketch has been greatly condensed ; for 

 it is obviously impossible to deal with so complicated a 

 subject in a limited space, and all that can be done is to 

 give some indication of the climatic conditions, local 

 peculiarities, and character of the flora at each spot where 

 plant-bearing deposits are found. The thorny subject of 

 bygone alternations of climate is perforce discussed, for 

 it lies at the root of our inquiry. I have also been obliged 

 to deal with another equally vexed question, the submer- 

 gence or elevation of the land in Pleistocene times; for 

 this obviously has a most important bearing on the possible 

 survival of plants within our Islands. In discussing the 

 past climatic changes, while giving the preference to the 

 evidence derived from remains of plants belonging to 

 existing species, I have not hesitated to supplement this by 

 an appeal to other groups of organisms, or to inorganic 

 geology ; for an assemblage of Arctic mammals, a group 

 of Arctic or desert mollusca, a morainic deposit, or erratics 

 brought by floating ice in an Arctic sea, are as good 

 evidence of climate as a group of plants, and are often 

 discoverable in strata in which no plants are preserved. 



Perhaps it will be asked why, if the British flora is to 

 be treated from standpoints which involve a consideration 

 of climatic and geographic changes such as cannot be 

 merely local, a still wider view is not taken, and this flora 

 dealt with as a mere outlier of the Pala^arctic one ? To 

 this I may reply, firstly, that the fossil plants of the periods 



