86 THE HISTORY OF THE NORTH ATLANTIC 



the northern belt of many temperate species of plants, just as 

 now happens with the Arctic flora ; and when these were dis- 

 placed by colder periods, they marched southward along both 

 sides of the sea on the mountain chains. 



The same remark applies to northern forms of marine inver- 

 tebrates, which are much more widely distributed in longitude 

 than those farther south. The late Mr. Gywn Jeffreys, in one 

 of his latest communications on this subject, stated that 54 

 per cent, of the shallow-water mollusks of New England and 

 Canada are also European, and of the deep-sea forms, 30 out 

 of 35 ; these last, of course, enjoying greater facilities for 

 migration than those which have to travel slowly along the 

 shallows of the coast in order to cross the ocean and settle 

 themselves on both sides. Many of these animals, like the 

 common mussel and sand clam, are old settlers which came 

 over in the Pleistocene period, or even earlier. Others, like the 

 common periwinkle, seem to have been slowly extending them- 

 selves in modern times, perhaps even by the agency of man. 

 The older immigrants may possibly have taken advantage of 

 lines of coast now submerged, or of warm periods, when they 

 could creep round the Arctic shores. Mr. Herbert Carpenter 

 and other naturalists employed on the Challenger collections 

 have made similar statements respecting other marine inverte- 

 brates, as, for instance, the Echinoderms, of which the deep- 

 sea crinoids present many common species, and my own collec- 

 tions prove that many of the shallow-water forms are common. 

 Ball and Whiteaves 1 have shown that some mollusks and 

 Echinoderms are common even to the Atlantic and Pacific 

 coasts of North America ; a remarkable fact, testifying at once 

 to the fixity of these species and to the manner in which they 

 have been able to take advantage of geographical changes. 

 Some of the species of whelks common to the Gulf of St. 

 Lawrence and the Pacific are animals which have no special 

 1 Ball, Report on Alaska ; Whiteaves, Trans. R. S. C. 



