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stream (if we may so call it) which had been long flow- 

 ing in a north-easterly course across its surface. What- 

 ever be the length of the periods, however, during which 

 these counter migrations were going on, I think it 

 sufficient to state that I would refer them to epochs 

 altogether different, so that, accompanied as they may 

 have been by special geological phenomena, which, if 

 known, would in all probability become at once explana- 

 tory, we should be the less inclined to regard as absurd 

 what might appear at first sight difficult to understand. 

 In the case of the British Isles indeed, no less than five 

 of these distinct migratory eras have been assumed, and 

 specified*, by Professor Edward Forbes; therefore (what- 

 ever value be attached to his able and interesting theory) 

 I do not consider it necessary to apologize for requiring 

 at least two in behalf of this ancient Atlantic province. 

 Not to insist upon those of his faunas and floras which 

 are of a less evident, or more questionable, character, 

 he has at any rate proved, I think, almost to a demon- 

 stration, the westward progress of the great mass of our 

 British animals and plants, over a then unbroken land 

 (the upheaved bed of the glacial sea), from the central 

 Germanic plains; whilst the accurate calculations of 

 the late Mr. Thompson of Belfast, concerning the reptile 

 statistics of Ireland, England, and Belgium, respectively, 

 have succeeded in showing, with much presumptive rea- 

 son, how the formation of St. George' s Channel, before 



* Origin of the Fauna and Flora of the British Isles (in Mem. of 

 the Geol. Survey of Great Britain, vol. i. p. 336, A.D. 1846). 



