Lutroduction, 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 Palearctic one? To 
this I may reply, firstly, that the fossil plants of the periods 
