Geographical Distribution of Crustacea. 43 



said on the influence of temperature, and the restriction of 

 species to particular temperature regions. It is not doubted 

 that the species have been created in regions for which they are 

 especially fitted ; that their fitness for these regions involves an 

 adaptation of structure thereto, and upon this adaptation, their 

 characteristics as species depend. These characteristics are of 

 no climatal origin. They 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. 



The following questions come 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 

 repeated by creation in distinct and distant regions ? 



3. How closely may we recognize in climatal and other phy- 

 sical conditions, the predisposing cause of the existence of specific 

 genera or species ? 



With regard to the second head, migration, we should re- 

 member, 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 Crus- 

 tacea are much more generally diff'used. 



A direction and also a limit to this migration exist, (1) in the 

 currents of the ocean, and (2) in the temperature of its diff'erent 

 regions. Through the torrid zone, the currents flow mainly 

 from the east towards the west ; yet they are reversed in some 

 parts during a certain portion of the year. But this reversed 

 current in the Pacific 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 temperate 

 regions in these latitudes are more numerous than in the tropics, 

 and species might readily be wafted to uncongenial climates, 

 which would be their destruction ; in fact they could hardly 

 escape this. Moreover, such seas are more boisterous than those 

 nearer the equator. Again, these waters are almost entirely bare 

 for very long distances, and not dotted closely with islands like 

 the equatorial Pacific. 



In the northern hemisphere, on the eastern coasts especially, 



