1582 CRUSTACEA. 



conditions, the predisposing cause of the existence of specific genera 

 or species ? 



With regard to the second head, migration, we should remember, 

 that Crustacea are almost wholly maritime or marine ; that marine 

 waters are continuous the globe around ; and that no seashore 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 are often to a considerable extent peculiar to particular groups 

 of islands, while the Crustacea are much more generally diffused. 



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

 rents of the ocean, and (2) in the temperature of its different 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 uniform and constant. Here is a means of east- 

 ward migration in the middle and higher temperate regions. But 

 the temperature 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 equa- 

 torial Pacific. 



In the northern hemisphere, on the eastern coasts especially, there 

 are warm currents from the south and cold currents from the north. 

 The former overlie the latter to a great extent in the summer and 

 may aid southern species in northward migrations. Cape Hatteras is 

 nearly the termination of the summer line of 70° (see Maury's Chart), 

 a temperature which belongs to the subtorrid region in winter. On 

 the China coast, at Macao there is a temperature of 83° in July, and 

 in the Yellow Sea, of 78° to 80°. But such northward migrations as 

 are thus favoured, are only for the season; the cold currents of the 

 winter months destroy all such adventurers, except the individuals of 



