Geographical Distribution of Crustacea. 355 
. The migration of species hi island to island through the 
tropical Pacific and East Indies may be a possibility; and the 
tree + 
ters is congenial through all this range, and the habits of many 
canola although they are not voluntarily migratory, seem to 
admit of it. ‘The species which actually have so wide a range 
are “ah Maioids (which are to a considerable extent deep-water 
Species), but those of aa penis and some, as Thalamita ad- 
mete, are swimming s 
Wi. The fact, that Sor ‘tov of the Oriental ms 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. 
“ogee the diffusion of Pacific tropical species to the West- 
ern ican coast is prevented, as already observed, by the 
onward aioe of the tropical currents, and the cold waters 
that bathe the greater part of this coast. 
en we compare the seas of Southern Japan and Port 
Natal and find species common to the two that are not now ex- 
isting in the Indian Ocean or East Indies, we hesitate as to mi- 
gration being a sufficient cause of the distribution. It may, 
however, be said that — of such species westward through 
the Indian Ocean ma iy h casionally taken place; but that 
only those individuals dest were carried during the season quite 
through to the swbéorrid region of the South Indian Ocean (Port 
Natal, etc.), survived and reproduced, the others, if continuing 
to live, so 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 ; bat whether this view will 
actually sag for the resemblance in species pointed out is 
:, 
2 MY. 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 natalensis, 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 thor- 
oughly acquainted with the species of the tropics, and that facts 
_ may hereafter be dieoovaed that will favor this view. The 
identical ae are of so peculiar a character that we deem this 
improba' 
¥: The ‘existence of the Plagusia tomentosa at the southern 
extremity of Africa, in New Zeal land, and on the Chilian coasts, 
may perhaps be due to migration, and especially as it isa south- 
