Geof/rnphical Distribution of Crustacea. 45 



of certain genera that arc clsewliere peculiarly British, or com- 

 mon only to Britain and America. 



13. An identity in certain species of Eastern and Western 

 America. 



The foiiowin'^ are the conclusions to which we are led by the 

 facts : — 



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

 tropical PaciHc 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 congenial through all this range, and the habits of 

 many Crustacea, although they are not voluntarily migratory, 

 seem to admit of it. The sjjccies which actually have so wide a 

 range are not ]\Iaioids (which are to a considerable extent deep- 

 water species), but those of the shores ; and some, as Thalamita 

 admete, are svvinnniug 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 oljserved, by the west- 

 ward direction of the trojucal currents, and the cold waters that 

 bathe the greater jjart of this coast. 



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

 Natal, and find species conmion to the two that are not now 

 existing in the Indian Ocean or East Indies, we hesitate as to 

 migration being a sufficient cause of the distribution. It may 

 how^ever be said, that driftings 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, &c.), survived and re])roduced ; the others, if continuing 

 to live, soon running out under the excessive heat of the inter- 

 mediate 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 Ga- 

 lene natnlensis, not a species found in any part of the torrid re- 

 gion, 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 



