GEOGRAPHICAL DISTRIBUTION OF CRUSTACEA. 1431 



Using these results as a key for comparison we at once perceive the 

 great influence of the oceanic movements on climate and on the geo- 

 graphical distribution of marine life. 



The polar current of the Southern Atlantic has a more northward 

 course in mid-ocean than that of the Pacific. It consequently bears 

 up the isocryme of 35° F. to the parallel of 50°, six degrees above the 

 mean. The effect on the other isocrymes of the Atlantic is very 

 remarkable. We perceive in the first place that the most southern 

 point of each of these isocrymes is not far from the mean position of 

 the same isocrymes in the Pacific, while the most northern point of 

 each is ten to twenty-five degrees farther north. Taking the position 

 of the isocrymes of 68° and 74° F., where they cross the meridian of 15° 

 west, as the mean position for this ocean, we find that the former is eight 

 degrees in latitude farther north than 68° F. in the South Pacific; and 

 the mean for the latter is in 7° south, while for the same in the 

 Pacific it is 20° south, making a difference of thirteen degrees. The 

 effect of the cold southern waters is consequently not along the 

 African coast alone, but pervades the whole ocean. It is hence 

 obvious, how utterly untenable the common notion, that the tropical 

 current from the Indian Ocean is the same which flows up the west 

 African coast. With a temperature of 56° south of Cape Town, it 

 would be wholly incapable of causing the great deflections for the 

 whole South Atlantic which have been pointed out. It combines with 

 the polar current, but does not constitute it. The facts thus sustain 

 the opinions long since brought forward by the distinguished meteor- 

 ologist Mr. Wm. C. Kedfield, that the currents flowing north along 

 the African and South American coasts are alike true polar or cold 

 temperate currents.* 



We may now turn to the North Atlantic. In this part of the ocean, 

 the mean positions of the isocrymes of 74° and 68° F., are near the 

 normal positions deduced from the Pacific. The line of 62° F. is in a 

 somewhat higher latitude, the mean position, excluding the eastern 

 and western deflections, being near the parallel of 36°. The line of 

 56° F. has the parallel of 42 h° north for its mean position over the 

 middle of the ocean, which is five and a half degrees above the normal 

 in the Pacific. The line of 50° has in the same manner for its mean 

 position over the mid-ocean, the parallel of 47£°, or again five and a 



* American Journal of Science, xlv. 299, 1843. 

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