58 ON THE LAND AND FRESH-WATER 



first propounded this theory, which has since been so ably advo- 

 cated by Dr. Dawson. Throughout all Canada, at any rate east 

 of the Niagara escarpment, we find, often at considerable heights 

 above the level of the sea, stratified deposits of sand and clay, full 

 of marine shells etc, generally of species which still inhabit the 

 Gidf of the St. Lawrence. These have been so carefully and ably 

 described by Dr. Dawson, that I need here do little more than 

 refer to his papers on this subject. It seems pretty clearly 

 proved that, at the time when these deposits were formed, the 

 whole of Lower Canada was submerged beneath the ocean, with 

 only the very highest points of the land left high and dry. 



To explain the great cold which is supposed to have obtained over 

 temperate Europe during the post-pliocene period, it has been ably 

 and ingeniously suggested that at the time of the general depres- 

 sion of the land, the isthmus of Darien, or part of it at least, was 

 submerged and the direction of one of the great currents of the 

 gulf stream consequently changed. Thus the warm current which 

 now washes the Western shores of Great Britain, then, it is urged, 

 ran up the west coast of north America ; while the cold current now 

 washing the mainland of Labrador, then flowed around the small 

 area of Europe left unmerged. When the re-upheaval of the land 

 took place, the isthmus of Darien would form an impassable bar- 

 rier against ocean currents, and would tend to produce the pre- 

 sent state of things. Of later years we have obtained a few more 

 facts bearing directly on this theory. Mr. Woodward, quoting 

 the views of Prof. C. B. Adams, states in his Manual, in 1856, that 

 only one marine shell (Purpura patula) is common to both sides of 

 the isthmus. But on referring to Mr. Carpenter's able report on the 

 mollusca of the west coast of North America, (Reports of the Brit- 

 ish Association for the Advancement of feience, 1857,) we find very 

 different views entertained. Thus he gives a list of thirty-five species 

 which unquestionably ■ live both on the Atlantic and Pacific shores* 

 To these he adds twenty-four species which are probably common to 

 both sides, and forty-one species inhabiting the same area, which he 

 considers " really separated but by slight differences." It is to be 

 remarked that our knowledge on these points is so limited, that 

 when large series have been procured, many species now separated, 

 may be considered identical. And from later sources, we learn that 

 some species, not included in this Report yet inhabit both oceans. 

 (A series of marine shells collected at Mazatlan by Mr. Moores of 

 Columbus, Ohio, was exhibited to support this view.) Further 



