284 PERIODS OP JUNCTION AND SEPARATION, chap. xiv. 



feet in its greatest depth, preceded the opening of the Straits 

 of Dover, or the final separation of Eiigkind from the Con- 

 tinent. This he inferred from the present distribution of 

 species both in the animal and vegetable kingdoms. Thus, 

 for example, there are twice as many reptiles in Belgium as 

 in England, and the number inhabiting England is twice 

 that found in Ireland. Yet the Irish species are all com- 

 mon to England, and all the English to Belgium. It is there- 

 fore assumed that, the migration of species westward having 

 been the work of time, there was not a sufficient lapse of ages 

 to complete the fusion of the continental and British rep- 

 tilian fauna, before France was separated from England and 

 England from Ireland. 



For the same reason there are also a great number of birds 

 of short flight, and small quadrupeds, inhabiting England 

 which do not cross to Ireland, the St. George's Channel 

 seeming to have arrested them in their westward course.* 



The depth of the St. George's Channel in the narrower 

 parts is only 360 feet, and the English Channel between 

 Dover and Calais less than 200, and rarely anywhere more 

 than 300 feet; so that vertical movements of slight amount 

 compared to some of those previously considered, with the 

 aid of denuding operations or the waste of sea-cliffs, and the 

 scouring out of the channel, might in time effect the insula- 

 tion of the lands above alluded to. 



Time required for successive Changes in Physical Geography iji 

 the Post-Pliocene Period. 



The time which it would require to bring about such 

 changes of level, according to the average rate assumed at 

 p. 58, however vast, will not be found to exceed that which 



* E. Forbes, Fauna and Flora of British Isles; ^Memoirs of Geological Survey, 

 vol. i. p. 344, 1846. 



