88 SEA-SIDE STUDIES. 



boring is tlins supposed to be a combination of chemi- 

 cal and mechanical agencies. There is, however, one 

 little difficulty in accepting this explanation, which 

 the author has overlooked, as speculators are wont to 

 overlook fatal objections : the existence of this acid 

 has yet to be proved ; its presence is indispensable to 

 the theory, but, unfortunately, the fact of its presence 

 is hypothetical. And when we have got tangible hold 

 of the acid, we must prove that, while it has the 

 property of attacking wood and rock, it has not the 

 property of attacking the calcareous shell of the 

 animal. 



Very different, and far more philosophical, is the 

 explanation of Professor Owen, whose opinion on all 

 points carries authority. He shows * that the com- 

 bined action of the muscular disc and the valves of 

 the shell will produce the phenomenon. It may be 

 paradoxical, and you will probably shake a dubious 

 head on seeing the cavities bored by these Molluscs, 

 and on being told that the^soft muscular disc of the 

 animal perforated them ; you have no conception that, 

 by licking limestone with never so much energy, you 

 could wear it away ; and yet, as Owen quietly remarks, 

 "it is certain that the perpetual renewal of a softer 

 substance will render it capable of wearing away a 

 harder one, subject to the friction of such softer sur- 

 face, and not like it susceptible of being repaired." 

 Yes, here lies the whole mystery : the soft muscular 

 disc is perpetually renewed, and the hard limestone 



* Lectures on Comparative Anatomy, i. 520. 



