88 SAXICAVOUS MOLLUSCA — THEIR INFLUENCE. 



the winter season. But the hosts of the Teredo are in mo- 

 tion : thousands of little augers are applied to the floating 

 harrier, and attack it in every direction. It is perforated, it 

 is lightened, it becomes weak ; it is dispersed, or precipitated 

 to the bottom ; and what man could not effect, is the work 

 of a worm. Thus it is that nothing is made in vain; and 

 that, in physics, as well as in morals, although evil is in- 

 termingled with good, the good ever maintains a predo- 

 minancy."* 



The Pholades, the Saxicavae, and Lithodomi, with habits 

 analogous to the Teredo, excavate their cells mostly in lime- 

 stone rocks and indurated clay, and in this manner contribute 

 to alter the configuration of the shores, f If you confine 

 your attention to the operations merely of those species 

 which are found on the coasts of Britain, and if, as one is apt 

 to do, you measure their influence by those which are work- 

 ing under your own observation, that influence you will infer 

 to be very slight ; but it is by slow and imperceptible steps 

 that all the great changes in the material world are effected ; 

 and to do justice to the lithophagous mollusca, you must 

 multiply the extent of their disintegrating excavations during 

 the brief span of your own life by the centuries of the world's 

 age, and your own little domain by the millions of miles 

 which engirdle the sea, along which their colonies are spread. 

 Their disintegrating influence must have been at all times, 

 and must continue to be, considerable ; for it is not merely 

 by their own excavations that the opposing rocks are reduced, 

 but through them water is admitted into their interior, and 

 aided by its macerations — its varying expansions under vari- 

 ous temperatures — the ceaseless wearing away of this addi- 

 tional agent, while it flows or percolates through new channels 

 opened to its access — the rock is speedily rubbed down into 

 an impalpable dust, or broken up into loose fragments. Con- 

 sider the result : the outline of the shore is altered, a barrier 

 to the tide removed, and perhaps some inroad is made on the 

 soil ; but the limestone thus triturated to powder is carried 

 off with the ocean wave, and, in a course which it is not diffi- 

 cult to follow, becomes absorbed by the myriads of animals of 

 every class, which again consolidate and convert it into coral 

 vegetations, crusts, and shells, — to become in their turn, at 

 some distant date, the foundation of a barrier of a future 



* Good's Book of Nature, i. 265. To the same purport see Sellius de 

 Tered. Mar. 175. 



t " Perforat — Teredo ligna, ut destruantur ; queuiadmodum Pholades, et 

 Mytili lithophagi petras, ut solvantur." Linn. Syst. 1069. 



