CONSPICUOUS EFFECTS OF CUREENTS. 265 



In the seventli chapter we have already seen that not 

 individual corals only but whole reefs are influenced in the most 

 decided manner by two forces, i.e. the strength and the direc- 

 tion of constant currents impinging on them. Their favourable 

 development depends, no doubt, on other things, as the warmth 

 of the water, its chemical composition, the accidental mingling 

 of species, &c. ; but all these influences, in my opinion, sink into 

 the background in comparison with currents ; for though they 

 may impede or even destroy the existence of the polyps, they 

 never, so far as I can see, force them to develope in any par- 

 ticular direction. Now this, as I have amply explained in 

 Chapter VII., is in an eminent degree the effect of constant 

 currents. Moreover in this investigation it is not our business 

 to determine whether corals can thrive, or even grow at all, in 

 this or that particular spot, but exclusively to decide whether 

 the forms of particular reefs, under circumstances of unhindered 

 growth, can be explained by known causes. 



We must remember that wherever constant and deep 

 ciirrents impinge on a coast at an angle, the reef will inovi ■ 

 tably grow upwards perpendicularly if the force of the current 

 is sufficient ; and, on the other hand, that many species of coral 

 have a tendency to grow equally in every dii'ection, as far as 

 circumstances allow, so long as shallow currents flow horizon- 

 tally over them. 



The high seas, the open ocean, exhibit both these modes of 

 motion of water in the most conspicuous degree. Late researches 

 have shown us that strong currents often flow at great depths ; 

 these constant currents maintain the same direction the whole 

 year through, though their force may vary ; even those arising 

 from the ebb and flow of the tide vary in strength under the 

 influence of the prev.iiling winds, but never, or rarely, in 

 direction. Besides these currents flowing at great depths there 

 are, in the second place, quite snperflcial ones, which are some- 

 times variable, particularly if they are afiected by the prevalent 

 winds, or sometimes very constant, as in the case of the drift 

 in great seas. 



The Pelew Islands lie, as is well known, within the region 

 of the north equatorial current flowing from east to west in the 



