16 
ORIGIN OF LIFE IN AMERICA 
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. Dali 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 ” (Liftor ina 
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. Ashworth f 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 Nathorst$ that a portion of 
* Morse, E. S., “Dispersion of certain Mollusks,” p. 8. 
t Ashworth, J. II., Catalogue of Chaetopoda part I. 
f Nathorst, A. G., “ Pflanzengeographie der Vorzeit,” p. 267. 
