Origin of ihe Geographical Distribution of Crastaca. 
The origin of the existing distribution of species jn this de- 
partment of zoology deserves attentive consideration. — 'T'wo 
great causes are admitted by all, and the important question 1s, 
how far the influence of each has extended. "The first, is origi- 
nal local creations ; the second, anisration. 
Under the first head, we may refer much that we have already 
said on the influence of temperature, and the restriction of spe- 
cies to partieular temperature regions. [It is not doubted that 
the species have been created in regions for which they are es- 
pecially fitted ; that their fitness for these regions involves an 
adaptation of structüre thereto, and upon this adaptation, their 
characteristics as species depend. "'lhese characteristics are of 
no climatal origin. 'l'hey are the impress of the Creator's hand, 
when the species had their first existence in those regions calcu- 
lated to respond to their necessities. 
''he following questions come up under this general head :— 
1. Have there been local centres of creation, from which 
groups of species have gone forth.by migration ? 
2. Have genera only and not species, or have species, been re- 
peated by creation in distinct and distant regions? 
3. How closely may we recognise in climatal and other physi- 
cal conditions, the predisposing cause of the existence of specific 
genera or species ? 
With regard to the second head, migration, we should remem- 
ber, that Crustacea are almost wholly maritime or marine; that 
marine waters are continuous the globe around ; and that no sea- 
shore species in zoology are better fitted than crabs for migration. 
'They may cling to any floating log and range the seas wherever 
the currents drift the rude craft, while the fish of the sea-shores 
will only wander over their accustomed haunts. Hence it is, 
that among the Pacific Islands the fishes of each group of islands 
are mostly peculiar to the group, while the Crustacea are much 
more generally diffused. 
.. À direction and also a limit to this migration exist, (1) in the 
eurrents of the ocean, and (2) in the temperature of its different 
regions. 'lhrough the 'lT'orrid zone, the currents flow mainly 
from ihe east towards the west; yet they are reversed in some 
parts during a certain portion of the year. But this reversed cur- 
rent in the Pacifie never reaches the American continent, and 
hence it could never promote migration to its shores. Again, 
beyond 30? or 35? of north or south latitude, the general course 
of the waters is from the west, and the currents are nearly uni- 
form and constant. Here is a means of eastward migration in 
the middle and higher temperate regions. But the temperature 
regions in these latitudes are more numerous than in the tropics, 
Fon Distribution of Crustaeoa. SEOGER 3p 
4 ha 
