THE BIRDS OF THE GREEN LANES. 105 



British mollusca migrated to seas further south, to 

 return to British areas when the cold had departed. 

 Beds of fossil shells, of the same species as those 

 now living in our seas, are found on the shores of 

 the Mediterranean. They no longer live in the 

 neighbourhood, but we know from the contents of 

 our English "Crag" formations, that they were 

 British before the commencement of the Glacial 

 epoch, just as we know they live in British seas 

 now. Again, arctic mollusca and arctic animals, 

 such as the reindeer, glutton, &c., dwelt in this 

 country during the cold period in question, thus 

 replacing the original inhabitants which the cold 

 had driven further to the south. Arctic birds would 

 doubtless come with the rest of the fauna, to depart 

 afterwards, when they had become attached to these 

 latitudes, in order to exchange a rigorous for a 

 temperate winter. In the case of land animals and 

 marine mollusca, we have their remains to testify to 

 the geographical changes. But we cannot expect 

 any geological deposit to chronicle a similar origin 

 of migratory habits on the part of birds. Arctic 

 plants still live on our high mountains, and arctic 

 mollusca in the deeper and colder parts of our seas, 

 both to add their testimony to the general weight oi 

 evidence in favour of the forced climatal movements 

 of organic beings. In this way, therefore, we think 

 a knowledge of the general physical and vital con- 

 ditions of the Glacial period will throw light on, if 

 not thoroughly account for, those singular habits 



