Geographical Distribution of Crustacea. 99 
1l. "he occurrence of a large number of identical species in 
the British. seas and. the Mediterranean ; and also in these seas 
and about the Canary Islands. 
12. 'T'he occurrence of closely allied, if not identical, species 
(as of Palemon) in New Zealand and the British seas; and also - 
of certain genera that are elsewhere peculiarly British, or com- 
mon only to Britain and America. 
13. An identity in certain species of Eastern and Western 
America. | . 
'T'he following are the conclusions to which we are led by the 
facts.— 
I. ''he migration of species from island to island through the 
tropical Pacific and East Indies may be a possibility ; and the - 
same species may thus reach even to Port Natal in South Africa. 
''he currents of the oceans favor it, the temperature of the wa- 
ters is congenial through all this range, and the habits of many 
Crustacea, although they are not voluntarily migratory, seem to 
admit of it. 'l'he 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 T'Àalamita ad- 
«eie, 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 West- 
ern 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 ex- 
. jsting in the Indian Ocean or East Indies, we hesitate as to mi- 
. gration being a sufficient cause of the distribution. lt may, 
however, 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, etc.), survived and reproduced, the others, if continuing 
tolive, 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 G'a- 
