Royal Physical Society. 13 



grooved surfaces, and the boulder clay, a period during which some of 

 their hardier cogeners were very abundant. And the catastrophe which 

 has just destroyed them in such numbers shows in part how this passage 

 in our geologic history may have taken place. 



Such a depression of a £ew hundred feet of the North American conti- 

 nent as that suggested by Mr Hopkins, would have the effect of diverting 

 the course of the great Gulf Stream into what is now the valley of the 

 Mississippi, and of sending it northwards over the Lake District into 

 Hudson's Bay and the Polar Seas. The heating agent, which has been 

 said to throw as much caloric in a single day into the Atlantic as would 

 raise the temperature all over France and Britain from the average of 

 that of winter to that of summer, would be lost, in consequence, to Eu- 

 rope ; and, left to the natural effects of our high latitudinal position, 

 whole races of the existing molluscs would die in our seas. Scenes such 

 as the one which we witnessed during the recent frost would occur with 

 every returning winter, until only the hardier shells would continue to 

 survive, and gradually and slowly, northern shells, not now living on our 

 coasts, or occurring in but scattered patches and outliers, such as Panopea 

 Norvegica, Tellina proximo,, Pecten Islandicus, and Astarte arctica, 

 would take the places of the perished ones. Our Fauna would become a 

 sub-arctic one, like that which now lives in the same parallel on the coast 

 of Labrador, or like that whose remains we find locked up in the Pleisto- 

 cene deposits of Banffshire, or in the boulder clays of Caithness. And 

 then, an elevation of the American valley to its present level, or to a 

 level approximating its present one, would again give us back the Gulf 

 Stream. A reverse process would take place among our molluscs ; the 

 sub-arctic ones would gradually die out in the over warm water, and 

 shells of the same species with those previously killed by the cold would 

 gradually propagate from the southern localities, to which they had been 

 restricted, and occupy their old areas as before. And such, judging from 

 the data furnished by our later deposits, from the Red Crag downwards, 

 seems to have been the geologic history of northern and western Europe. 

 Independently, however, of these views, though he could entertain no 

 doubt that they are ultimately to prevail, any catastrophe illustrative of 

 the extinction of species must be of interest to the geologist. To the 

 mystery of creation he could not attain ; but the twin problem of extinc- 

 tion appears to be a solvable one. That law through which, judging 

 from the past, all species are as certainly destined to die as all individuals, 

 seems fairly to belong to the field of experience. He had already referred 

 this season, in one of his communications to the Society, to that myste- 

 rious disease which, selecting one of our most useful vegetables, destroyed 

 it over wide areas, and by billions of individual plants, as mayhap illus- 

 trative of one of those agents of death through which whole species are 

 exterminated ; and the late severe frost, which, equally, though less re- 

 ssrictively, selective of species, has strewed our shores with heaps of dead 

 shell-fish, seems not less illustrative of another and better appreciable 



