2J2 J. D. Dana on an Isothermal Oceanic Chart, 



zone than in the Torrid, it being 20° F. in the latter, and 

 33 P. in the former ; and this should be a cause of a greater 

 variety of genera in the latter for the same number of 

 species. 



In the Torrid zone the Subtorrid Region has nearly one- 

 third the surface of the Torrid Region, and not one-fourth 

 as much coast-line, facts which should be regarded in com- 

 paring the number of species of the two. 



We add here a few brief remarks, in a popular way, on 

 the origin of the peculiar forms and positions presented by 

 the isothermal lines of the ocean. The great currents of the 

 globe are admitted to be the causes that produce the flexures 

 and modify the courses of these lines. These currents are 

 usually of great depth, and consequently the deflecting land 

 will be the deeply seated slopes off a coast, beyond ordinary 

 soundings. 



The eastern coasts of the continents either side of the 

 equator feel the influence of a warm equatorial current, 

 which flows westward over each ocean, and is diverted north 

 and south by the coasts against which it impinges, and more 

 or less according to the direction of the coast. 



The western coasts of the continents, on the contrary, re- 

 ceive a strong extra-tropical or polar current. In the south- 

 ern oceans, it flows from the westward, or southward and 

 westward, in latitudes 45° to 65° south, and is brought to 

 the surface bv the submarine lands or the submarine slopes 

 of islands or continents ; reaching the continents of Africa 



i. 47. The first is the zone from the equator to the parallel of 2*°, the second, 

 from 2*° to 5°, and so on. 



2F . 





5 





n • 





10 





12* . 



. 



15 





17* , 





20 



. 



22* . 





25 





27* . 





30 





The zone 



from 60 



" 



70 



<< 



HO 



3,239,296 



32*° 





3,232,800 



35 





3,220,496 



37* 





3,202,048 



40° 





3,177,472 



42* 





3,146,912 



45 





3,110,320 



47* 





3,067,808 



50 





3,019,472 



52£ 





2,962,176 



55 





2,905,632 



57* 





2,840,368 



60 





to 70 has th 



i area, 





to so 



" 





to 90 «< 



u 





2,769,696 

 2,693,760 

 2,612,688 

 2,526,624 

 $435,776 

 2,340,256 

 2,241,280 

 2,136,128 

 2,027,840 

 1,915,696 

 1,767,168 

 1,680,704 

 5,466,992 

 3,350,064 

 1,128,114 



