272 MB. A. B. HUNT ON THE INFLUENCE OF 



In the foregoing pages I have called attention to the facts that 

 wave-currents affect the bottom in shallow water, and that they 

 are a source of danger to the fauna inhabiting such water ; and I 

 have adduced a few examples from my own limited experience of 

 animals that are specially adapted to withstand the attacks of 

 such wave-currents. It remains for me, in conclusion, to show 

 how, under the influence of wave-currents, the variation of species 

 may be promoted, and their local extinction brought about. 

 The common spinous cockle, Cardium echinatum, will serve to 

 illustrate the manner in which wave-currents may influence 

 variation. 



This species varies much in form and in length of spines. Mr. 

 Gwyn Jeffreys describes two distinct varieties in addition to the 

 type. The sand in which G. echinatum lives is also variable ; it 

 varies in size and character of grain, in specific gravity, and in 

 the amount of its admixture with mud. Some localities are 

 exposed to wave-currents, some to tidal currents, and some to 

 both combined. "Wave-currents acting alone, and giving rise 

 to no general forward movement, sort and arrange the materials 

 composing the bottom. "Wave-currents in conjunction with tidal 

 currents tend to produce a general motion of the bottom-deposit, 

 if movable at all, causing denudation at one place and accumula- 

 tion at another. A shell may withstand the local rearrangement 

 of deposit caused by wave-currents, by passive resistance, moored 

 in its locally disturbed bed ; but against the more widely-spread 

 motion caused by wave and tide combined more active resistance 

 is necessary. 



Cardium echinatum owes its safety to its powers of burrowing 

 and to its spines, which, curved in the direction of its tubes, offer 

 the least resistance to the cockle's penetration of the sandj and 

 the greatest resistance to its dislodgment therefrom. When 

 pitted against wave-currents the spines are of use to their owner, 

 but when pitted against wave and tide together they must be pre- 

 judicial, as, notwithstanding their curvature, they cannot fail to 

 offer great resistance to the animal in burrowing. Similarly, a 

 shell of globular form will serve the purpose of a mollusk that 

 relies on its powers of maintaining a fixed position, whereas a 

 shell of a more compressed form will be better suited to one that 

 depends for safety on its power of penetration. When the fry of 

 Buch a variable species as G. echinatum is spread in countless mil- 

 lions over an area affected by wave-currents, it is reasonable to 



