&* " 
P. vM , 
A0 G'eopraphical Distribution of. Crustacea. 1 
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- 
:eughly acquainted with the species of the tropies, and that facets 
may hereafter be discovered that will favor 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 south- 
ern species, and each of these localities is within the subtemper- 
ate region. We are not ready however to assert, that such jour- 
neys as this range of migration implies are possible. 'l'he oceanic 
currents of this region are in the right direction to carry the spe- 
cies eastward, except that there is no passage into this western 
current from Cape Horn, through the Lagulhas current, which 
flows the other way. t appears to be rather a violent assump- 
tion 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 boisterons passage, and finally have had 
a safe landing on the foreign shore. "l'he distance from New 
Zealand to South America 1s five thousand miles, and there is at 
present not an island between. 
VI. Part of the diffieulty 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 commu- 
nication once had place; and this bare possibility weakens the 
force of the argument used above against migration. Yet.as it 
is wholly an assumption, we cannot rely upon it for qtio 
that migration has actually taken place. iiw 
VII. "The existenee of the same species on the east. init | 
coasts of America, affords another problem, which migration . ca 
not 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 sup- 
position might be drawn from the very small number of species — | 
that are identical on the two sides, and the character of these spe- 
cies. Libinia spinosa occurs at Brazil and Chili, and has. not 
been found in the West Indies.  Leptopodia sagittaria, another 
Maioild, 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 temperature 
A 
