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54 PHYSICAL GEOGRAPHY, OF THE SEA. 
the analogous trees grow 3° or 4° nearer to the pole, and the 
aboriginal tribes go naked as far to the north as 52°, a simplicity 
of toilet that would but ill suit the Esquimaux of Labrador. 
Besides their beneficial influence on different climates the 
ocean-currents tend to equalise, or to maintain the equilibrium 
of the saline composition of sea-water, and thus secure the 
existence of numberless marine animals. Their movements 
also contribute to the formation of sand-banks, where at certain 
seasons legions of fishes deposit their spawn and invite the per- 
secutions of man. 
The rapidity of currents is very different, but always impor- 
tant enough to be taken into account by navigators. The well- 
informed seaman makes use of them to traverse wide spaces 
with greater rapidity, and, after an apparently circuitous course, 
arrives sooner and more safely at his journey’s end than the 
ignorant steersman, who vainly endeavours to strive agaiust 
their power. 
Pavonia Lactuca, with Polypes 
in Natural Position. 
