280 THE INFLUENCE OF INANIMATE SUEEODNDINGS. 



perfectly incapable, of swimming against the feeWest stream. 

 The only invertebrate animals which are able to overcome 

 perhaps the strongest currents are the Cuttle fishes. 



The well-known wealth of forms in the Mediterranean and 

 in the Red Sea owes its origin, certainly in great part, to the 

 action of the constant marine currents. Both these seas are 

 connected with the ocean only by narrow straits through 

 which a superficial current incessantly flows in. The strength 

 of these currents may vary with the time of year and the direc- 

 tion of the prevailing winds, but their direction is invariable the 

 whole year through. Hence all the animals drifting on or just 

 below the surface, when once they have been carried in through 

 these narrow straits, cannot easily get back to the open ocean, 

 and so all the forms that never sink below a certain inconsider- 

 able depth must remain in the inland sea, and only those few 

 species or individuals which reach the deeper return current 

 and do not leave it can be in a position to be borne back by 

 it to the ocean. Consequently both these seas, by reason of the 

 inflowing surface cuiTents, are a sort of trap ; everything can 

 get in, but nothing can get out again ; thus it is inevitable — 

 and it is actually the case — that a vast accumulation of species 

 as well as of individuals occurs in thes'e seas, wherever the 

 other necessary conditions for the existence of the individual 

 forms exist. 



Another result which may be directly referred to marine 

 currents —in combination, of course, with other influences — is the 

 wide distribution of such marine animals as have free-swimming 

 larvre. If, for instance, we compare the marine mollusca of the 

 Red Sea with those of the Philippines, their extensive resem- 

 blance strikes us at once; nay, even those of the Western 

 Pacific closely agree with them both in several respects. In 

 strong contrast with this state of things is the fact that the land 

 mollusca of Eastern Africa, the Moluccas, the Philippines, and 

 western islands of the Pacific, exhibit scarcely a single genus 

 which is common to them all, and the species — irrespective of 

 a few unimportant forms probably introduced by man — are 

 totally different in all these provinces. 



This resemblance between the fauna of extensive marine 



