324 On an Isothermal Oceanic Chart, illustrating 
gical Provinces; and as regards marine animals, ocean-tempera- 
ture is the more prominent of these influences. Under tempera- 
mean, and also the varying action of currents which induce the 
changes, especially those occasional extreme results which are of 
decennial rather than annual recurrence. 
How far geological changes, by subsidence and elevation, have 
varied the distribution of the present races of animals, or given 
limits to zoological regions, is a point yet uninvestigated. ‘The 
conclusions that have been derived from this source are mostly 
a hypothetical character, and are to be received with distrust 
without a larger supply of evidence. It is easy to meet a diffi- 
culty by the supposition of a former union by dry land of regions 
now separate ; but it appears to us that better evidence is needed 
on such a point, than those derived from the zoological fact which 
is to be explained. : 
Along the various coasts, prominent capes are in general the 
limits of Zoological Provinces; and this fact is well shown in 
the chart of ocean-temperature. They are, as we have explained, 
the points where the cold or warm currents are turned off from a 
coast, and where therefore there is a sudden transition in the tem- 
perature. A striking example of this has been pointed out on both 
the eastern coast of North America, and western of South America, 
att 
56°, and 50°, and Cape Blanco, the meeting point of 68°, 62°, 
and (nearly) 74°. So also the east cape of Kast Australia, is the 
point of meeting of the isocrymes of 74° and 68°. At the south 
extremity of Africa, on the west coast of Asia, there are approxi- 
mations to the same fact. Cape Cod the southeast cape of New 
England, is a marked point in zoological geography, and the ter- 
mination of the isocryme of 44° F.; and the North Cape of the 
La Plata inside of Maldonado is another. 
We proceed to give an enumeration of the several Zoological 
Provinces to which we are led by the temperature regions adopted. 
It should be again observed, that the isocryme of 68° is the grand 
boundary of coral reefs, and of the larger part of the zoological 
life connected with them, and that the Torrid Zone and Coral- 
reef Zone of oceanic temperature are synonymous terms. 
We mention also the extent of the Provinces; and it will be 
found, that although seemingly numerous, few of them are under 
500 miles in length, while some are full 4000 miles. 
For zoological reasons which are explained in another place,* 
and which may be the subject of another communication to this 
Journal, we adopt for Marine Zoological Geography, three grand 
* The Author's Report on Crustacea. 
