818 



ISLAND LIFE. 



[part II. 



up of a freshwater shell (Unio picto7'um) off the mouth of the 

 English Channel between the fifty fathom and 100 fathom 

 lines, while in the same locality gravel banks with littoral shells 

 now lie under sixty or seventy fathoms water. ^ More recently 

 Mr. Gwyn Jeffreys has recorded the discovery of eight species 

 of fossil arctic shells off the Shetland Isles in about ninety 

 fathoms water, all being characteristic shallow water species, 

 so that their association at this great depth is a distinct indication 

 of considerable subsidence.^ 



Time of last Union with the Continent. — The period when 

 this last union with the continent took place was comparatively 

 recent, as shown by the identity of the shells with living species, 

 and the fact that the buried river channels are all covered with 

 clays and gravels of the glacial period, of such a character as 

 to indicate that most of them were deposited above the sea- 

 level. From these and various other indications geologists are 

 all agreed that the last continental period, as it is called, was 

 subsequent to the greatest development of the ice, but probably 

 before the cold epoch had wholly passed away. But if so 

 recent, we should naturally expect our land still to show an 

 almost perfect community with the adjacent parts of the con- 

 tinent in its natural productions ; and such is found to be the 

 case. All the higher and more perfectly organised animals are, 

 with but few exceptions, identical with those of France and 

 Germany ; while the few species still considered to be peculiar 

 may be accounted for either by an original local distribution, 

 by preservation here owing to favourable insular conditions, or 

 by slight modifications having been caused by these conditions 

 resulting in a local race, sub-species, or species. 



Why Britain is poor in Species. — The former union of our islands 

 with the continent, is not, however, the only recent change they 

 have undergone. There is equally good evidence that a consider- 

 able portion, if not the entire area, has been submerged to a depth 

 of nearly 2,000 feet (see Chap. IX. p. 168), at which time only 

 what are now the highest mountains would remain as groups 

 of rocky islets. This submersion must have destroyed the 



1 Quarterly Journal of Geological Society., 1850, p. 96. 

 ^ British Association Report, Dundee, 1867, p. 431. 



