16 



Hence they evidently wandered northward under different 

 geographical conditions than exist at present, and only main- 

 tain a slender hold in isolated localities, where they form 

 relict colonies. I assume that this northward advance of the 

 marine fauna took place in late Pliocene times when the 

 northern Atlantic was closed, and the temperature of the 

 ocean raised. As Dr. Dall wrote to me " the Pliocene all over 

 the northern hemisphere was a period of warmer sea water 

 than the Miocene or Pleistocene." My attention was also 

 recently drawn by Professor Morse* to the fact that even 

 littoral European species have extended their range across the 

 ocean to North America. As long ago as 1855 he received 

 specimens of the common European " Periwinkle " (Littorina 

 litorea) from Chaleur Bay in the Gulf of St. Lawrence. Later 

 on this 'mollusk was reported also to occur on the coast of 

 Nova Scotia and Labrador. It is steadily advancing now in a 

 southward direction, while it is becoming scarce in the north. 



Dr. Ashworthf tells me that the lugworm (Arenicola 

 marina) which is common on the west coast of Europe is also 

 met with on the coasts of the Shetland islands, the Faroes, 

 Iceland, Greenland, Labrador and Nova Scotia. It is absent 

 from the coasts of the Pacific Ocean. Hence he concludes 

 that a continuous shore-line formerly existed between the two 

 areas on each side of the Atlantic Ocean in which this worm 

 is now found. Dr. Ashworth believes that the extensive 

 stretch; of ocean at present existing between Europe and 

 America forms just as much a barrier to the dispersal of 

 the littoral lugworm as it does to that of the terrestrial Helix, 

 hortensis just alluded to. 



Commenting on this occurrence of European littoral marine 

 mollusks on the north-east coast of North America, Mr. B. B. 

 Woodward writes to me that no other theory than that of the 

 previous existence and subsequent rupture of a land bridge 

 in the direction indicated can satisfactorily account for the 

 present disjointed distribution of the two divisions of the 

 boreal fauna. 



It was assumed by Professor NathorstJ that a portion of 



* Morse, E. S., "Dispersion of certain Mollusks," p. 8. 



t Ashworth, J. H. , Catalogue of Chaetopoda part I. 



\ Nathorst, A. G., " Pflanzengeographie der Vorzeit," p. 267. 



