COOKE : ON GEOGKAPHICAL DISTRIBUTION. Ill 



somewhere just north of where the Gulf Stream leaves it, and thus 

 brings a cold-water fauna down to a point in the latitude of Lisbon, 

 and effectuall)^ prevents the tropical fauna from creeping any further 

 north. Thus Acmcsa testudinalis, Miill. (a cold-water species), 

 ranges as far south as New Jersey, in N. lat. 40°, while on the 

 European side it has not been found south of about 54° on the 

 east coast of England, and about 53° on tlie Irish coast, while hardy 

 circumpolar species like Macoma balthica, L., Ilya arenaria, L., 

 and Mytilus edulis, L., creep as far south as Hatteras and even 

 Georgia. 



It is interesting to observe tliat besides tlie migrants from the 

 sub-Arctic fauna southward, and from the tropical fauna northward, 

 the eastern shores of North America have a temperate fauna of their 

 own, which appears to be comparatively unaffected by the great change 

 of temperature which occurs at Cape Hatteras. For we tind a large 

 number of species, corresponding to the temperate element in our own 

 seas, which occur commonly between Cape Cod and Georgia or even 

 Florida. Possibly this faur.a may be considered to have taken up its 

 abode on these coasts before the present conditions of current became 

 lixed. Thus there are five species of Fulgiir, all of which occur in 

 Georgia, which may be regarded as their metropolis ; three of these 

 reach the West Indies and three Cape Hatteras, but two range 

 nortliward as far as Cape Cod. Nassa trivittata, Say, extends from 

 St. Augustine in North Florida to Nova Scotia, N. vibex, Say, from 

 Aspinwall to Cape Cod, iV. ohsoleta, Say, from Tampa to Nova Scotia. 

 Two muricidan species, both strongly characteristic of East American 

 temperate shores, are Urosalpinx cinerea, Say, and Eupleura caudata, 

 Say. The former ranges from Florida to Nova Scotia, the latter 

 from Florida to Cape Cod. Similarly, Asti/ris lunata, Say, ranges 

 from Turtle Harbour in West Florida to Cape Ann, and Anachis 

 avara, Say, from Florida Keys to Massachusetts I5ay. In all these 

 cases what may be called the indigenous fauna pass Avith ease 

 a barrier which proves so formidable to the northern and southern 

 migrants. 



Now let us compare the position on the western side of North 

 America. Our informatitm may not be quite so full, but the general 

 trend of distribution is plain. Here the tropical fauna of the Panamic 

 region, instead of being carried far northward along the coast by 

 a warm-water current, is checked by the far-reaching effect of 

 a stream of cold water. The Kuro Shio current, issuing from the 

 warm seas to the south of Japan, and crossing the North Pacific, loses 

 much of its warmth in the passage, and is very possibly reinforced by 

 cold water from tlie north. It impinges on the West American coast 

 about the latitude of Queen Charlotte Island (N. lat. 52°), and breaks 

 into two branches, the northern of which washes the coasts of North 

 Canada and Alaska, while the southern moves southward along the 

 coasts of Oregon and California. The effect of this cool current 

 sweeping southward must obviously be to keep back the northward 

 spread of the tropical species. The result is that the same genera, 

 Stromhus, Oliva, Cassis, Conus, etc., which were well represented up 



