Erratic Blocks in Plains. 235 



the formations which geologists have termed diluvium, and 

 whose various phenomena have hitherto been erroneously 

 attributed to one single cause, — currents. 



It appears to me probable, according to the facts which I 

 have been able to combine in considering this question, that 

 the organized beings of our epoch were created successively, 

 after the commencement of the retreat of the ice. Wherever 

 the surface of the ground made its appearance between the 

 glaciers, under the influence of a milder climate, — wherever, 

 yielding to the temperature, the ice produced pools of water, 

 — the development of organized beings might take place ; and 

 direct observation has already confirmed what the theory re- 

 quired. Mr Smith of Jordanhill was the first to point out in 

 the post-tertiary clays, which are superior to the till (that is to 

 say, which have been deposited posteriorly to the accumula- 

 tion of those masses of gravel and rolled blocks in the mud 

 under the ancient glaciers), numerous fossils of species that 

 no longer exist similarly associated on the neighbouring 

 coasts ; he has even ascertained the identity of some of those 

 shells with species which have hitherto been observed only in 

 the Arctic seas. A fact so unexpected did not fail to excite my 

 curiosity in a high degree, and 1 have ever since been un- 

 remitting in my endeavours to compare these fossils with 

 living species. Assisted by a collection of living species from 

 Greenland, which I owe to my friend Professor Eschricht 

 of Copenhagen, I have not only confirmed the first impres- 

 sions of Mr Smith, but have further found among the fossils 

 of these clays a much larger proportion of Arctic species than 

 could have been expected. Extending this species of research 

 to the most recent fossiliferous deposits of other parts of 

 Europe, I have every where met with a certain proportion of 

 species whose types no longer exist in a living state in the neigh- 

 bouring seas, but at 12° or 15° of latitude more to the north. 

 Thus, while the shells, which are now found in lat. 65° to 70° on 

 the coasts of Iceland and Greenland, where the mean tempera- 

 ture is several degrees below zero (32° F.), lived in lat. 55° to 60° 

 on the coasts of Scotland and of England, where the mean tem- 

 perature at present is -|- 8° (46°. 4 F.) ; the species of the coasts 

 of England and of the British Channel which now live in lat. 50* 



