46 Mr. J. D. Dana on the Origin of the 



thoroughly acquainted with the species of the tropics, and that 

 facts may hereafter be discovered 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 

 extremity 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 sub- 

 temperate 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 di- 

 stance 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 as- 

 sume 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 com- 

 munication once existed ; and this bare possibility weakens 

 the force of the argument used above against migration. Yet 

 as it is wholly an assumption, w^e cannot rely upon it for evi- 

 dence 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. We know of no evidence 

 whatever 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 sagittaria, 

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

 Canaries. 



VIII. The large number of similar species common to the 

 Mediterranean and British seas may be due to migration, as 

 there is a continuous line of coast and no intermediate tempera- 

 ture rendering such a transfer impossible ; and the passage 

 farther south to the Canaries of several of the species is not 



