1584 CRUSTACEA. 



The following are the conclusions to which we are led by the facts : 



I. The migration of species from island to island through the tro- 

 pical Pacific and East Indies may be a possibility; and the same 

 species may thus reach even to Port Natal in South Africa. The 

 currents of the oceans favour it, the temperature of the waters is con- 

 genial through all this range, and the habits of many Crustacea, 

 although they are not voluntarily migratory, seem to admit of it. 

 The species which actually have so wide a range are not Maioids 

 (which are to a considerable extent deep-water species), but those of 

 the shores ; and some, as Thalamita admete, are swimming species. 



II. The fact, that very few of the Oriental species occur in the 

 Occidental seas, may be explained on the same ground, by the barrier 

 which the cold waters of Cape Horn and the South Atlantic present 

 to the passage of tropical species around the Cape westward, or to 

 their migration along the coasts. 



Moreover, the diffusion of Pacific tropical species to the Western 

 American coast is prevented, as already observed, by the westward 

 direction of the tropical currents, and the cold waters that bathe the 

 greater part of this coast. 



III. When we compare the seas of Southern Japan and Port Natal 

 and find species common to the two that are not now existing in the 

 Indian Ocean or East Indies, we hesitate as to migration being a suf- 

 ficient cause of the distribution. It may, however, be said that drift- 

 ings of such species westward through the Indian Ocean may have 

 occasionally taken place; but that only those individuals that were 

 carried during the season quite through to the subtorrid region of the 

 South Indian Ocean (Port Natal, etc.), survived and reproduced, the 

 others, if continuing to live, soon running out under the excessive heat 

 of the intermediate equatorial regions. That they would thus run 

 out in many instances is beyond question ; but whether this view will 

 actually account for the resemblance in species pointed out is open to 

 doubt. 



IV. When further, we find an identity of species between the 

 Hawaiian Islands and Port Natal — half the circumference of the 

 globe, or twelve thousand miles, apart — and the species, as Oalem 

 natalensis, not a species found in any part of the torrid region, and 

 represented by another species only in Japan, we may well question 

 whether we can meet the difficulty by appealing to migration. It 

 may however be said, that we are not as yet thoroughly acquainted 



