AMERICAN AND WEST INDIAN FAUNA AND FLORA. 119 
On the Atlantic shores of the United States we have on a 
more extended scale a repetition of the phenomenon first ob- 
served in the Fzeróe Channel by the “ Lightning," of the dif- 
ference produced on the character of the fauna by a change of 
temperature. The “warm and the cold areas," as they have 
been termed there, find their parallel on our coast as seen in 
the extension of the West Indian fauna along the line of the 
continental slope bathed by the Gulf Stream, flanked on either 
side by the arctic fauna living in the colder water of the bottom 
of the Gulf Stream on the east, and in the cold arctic waters 
which bathe the continental plateau within the sixty-fathom line 
on the western edge of the area oecupied by this more tropical 
fauna. This shows in a remarkable way the powerful influence 
exerted by the action of the heated water of the Gulf Stream 
in extending to the northward, along the bottom, the range of 
the fauna of a more southern latitude. This action is not 
limited to the bottom, for on the surface also the character- 
istic pelagic fauna is carried northward. The larve of many 
of the Caribbean and Mexican animals which swim, find an 
abode as far north as the conditions of the water and the 
bottom will allow ; the adults may also, little by little, creep up 
northwards. 
This northern extension of the southern fauna has been care- 
fully studied by the United States Fish Commission along the 
southern coast of New England; their dredgings have proved 
the existence in this region of many animals known before only 
from Florida, and of others which were previously considered 
as characteristic tropical types. In fact, we have a northern ex- 
tension, off the southern coast of New England, of the West 
Indian and Gulf Stream fauna. The warm belt extending 
from sixty-five to one hundred and twenty fathoms has an 
equable temperature during the whole year, due to the action of 
the Gulf Stream, while on the inshore plateau arctic animals 
are found identical with those occurring at similar depths 
north of Cape Cod, which flourish only in a variable but cold 
temperature. Below the warm belt the temperature is uni- 
formly cold, and passes into that of the general abyssal region 
beyond it. The warm belt is but the northern extension of 
