GEOGRAPHICAL DISTRIBUTION OF CRUSTACEA. 1585 



with the species of the tropics, and that facts may hereafter be dis- 

 covered that will favour this view. The identical species are of so 

 peculiar a character that we deem this improbable. 



V. The existence of the Plagusia tomentosa at the southern extre- 

 mity of Africa, in New Zealand, and on the Chilian coasts, may 

 perhaps be due to migration, and especially as it is a southern species, 

 and each of these localities is within the subtemperate region. We 

 are not ready however to assert, that such journeys as this range of 

 migration implies are possible. The oceanic currents of this region 

 are in the right direction to carry the species eastward, except that 

 there is no passage into this western current from Cape Horn, through 

 the Lagulhas current, which flows the other way. It appears to be 

 rather a violent assumption that an individual or more of this species 

 could reach the western current from the coast on which it might 

 have lived ; or could have survived the boisterous passage, and finally 

 have had a safe landing on the foreign shore. The distance from New 

 Zealand to South America is five thousand miles, and there is at 

 present not an island between. 



VI. Part of the difficulty in the way of a transfer of species between 

 distant meridians might be overcome, if we could assume that the 

 intermediate seas had been occupied by land or islands during any 

 part of the recent epoch. In the case just alluded to, it is possible 

 that such a chain of interrupted communication once had place ; and 

 this bare possibility weakens the force of the argument used above 

 against migration. Yet as it is wholly an assumption, we cannot rely 

 upon it for evidence that migration has actually taken place. 



VII. The existence of the same species on the east and west coasts 

 of America, affords another problem, which migration cannot meet, 

 without sinking the isthmus of Darien or Central America, to afford 

 a passage across. As yet we know of no evidence that this portion of 

 the continent has been beneath the ocean during the recent epoch. 

 An argument against such a supposition might be drawn from the 

 very small number of species that are identical on the two sides, and 

 the character of these species. Libinia spinosa occurs at Brazil and 

 Chili, and has not been found in the West Indies. Leptopodia sagitr 

 taria, another Maioid, occurs at Valparaiso, the West Indies, and the 

 Canaries. 



VIII. The large number of similar species common to the Mediter- 

 ranean and British seas may be due to migration, as there is a con- 



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