220 J. D. Dana on an Isothermal Oceanic Chart, 



species was created may be ascertained by observing which 

 is most favourable for its development; and by this course 

 of investigation, we may find that almost every different lo- 

 cality has some species for which it is especially fitted. We 

 may thus shew, as far as reason and observation can do it, 

 that all regions, as a general thing, have had their own spe- 

 cial creations. 



We rather look to climatal influences, in all their various 

 kinds, directly and indirectly exerted, and united with height 

 or depth of site, and other geographical conditions, as giving 

 limits to Zoological Provinces ; and as regards marine ani- 

 mals, ocean temperature is the more prominent of these in- 

 fluences. Under temperature, the limits or extremes are to 

 be considered as well as the mean, and also the varying ac- 

 tion of currents which induce the changes, especially those 

 occasional extreme results which are of decennial rather than 

 annual occurrence. 



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 uninves- 

 tigated. The conclusions that have been derived from this 

 source are mostly of a hypothetical character, and are to be 

 received with distrust without a larger supply of evidence. 

 It is easy to meet a difficulty 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 shewn 

 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 sud- 

 den transition in the temperature. A striking example of 

 this has been pointed out on both the eastern coast of North 

 America, and western of South America, where several iso- 

 crymes meet, forming a kind of nodal point ; — viz., Cape Hat- 

 teras, the meeting point of the isocrymes of 62°, 56°, and 50°, 

 and Cape Blanco, the meeting point of 68 , 02 ', and (nearly) 

 71. So also the east cape of East Australia, is the point of 



