GEOGRAPHICAL RANGE. 161 



those exhibiting the mean temperature of the waters along 

 their course, for the coldest thirty consecutive days in any 

 season of the year, are given ; and which lines Mr. Dana 

 has shown are the most influential in governing the dis- 

 tribution of marine animals. At the isocryme of 68°, 

 Mr. Dana divides the torrid and sub-torrid zones from the 

 several temperate zones ; and at 44°, the temperate from 

 the sub-frigid and frigid zones ; but as no Cirripedes are 

 exclusively confined to these frigid zones, we may here dis- 

 regard them. From Mr. Dana's* table of the areas of the 

 torrid and temperate ocean-zones, on both sides of the 

 equator, it seems that they are nearly as 337 to 278, in 

 relative area ; and consequently, he remarks, that the marine 

 species in any class, if distributed equally over the two, would 

 be one fifth more numerous in the torrid than in the tem- 

 perate zones. Now of Cirripedes, taking all the orders, there 

 are at present known 147 species ; of these, 7 have doubtful 

 habitats, leaving 140 for comparison. Of these 140, nearly 

 one quarter, or 37, inhabit both the torrid and temperate 

 zones, as above defined ; 46 are found exclusively in the 

 torrid, and 57 exclusively in the temperate zones ; so 

 that the temperate zones, though less in area, and having, 

 proportionally, even a considerably lesser length of coast- 

 line, nevertheless have a preponderance in the number of 

 species. But it should be borne in mind, that there are 

 two great temperate districts, separated from each other by 

 one great torrid district ; and, inasmuch as the number 

 of species in any region seems to depend in some degree 

 on the isolation of the sub-regions, we might have ex- 

 pected (the other conditions now being, and the past condi- 

 tions having been alike), that the two great temperate 

 areas would have contained more species, perhaps doubly 

 more, than the single great torrid area. 



The proportional numbers, above given, are not very 

 widely different, whether we take separately the Balanidae, 

 the Lepadidse, or all together. Mr. Dana has shownf at 

 length, that generally amongst the Crustacea, the species 

 which he considers of highest rank, belong to the extra- 



* 'Crustacea: United States Exploring Expedition,' p. 1476 (corrected), 

 f Ibid., p. 1528. 



11 



