326 THE INFLUENCE OF INANIMATE SUKEODNDINGS. 



structure of the skeleton. But between a merely mechanical 

 modification of a bone during the lifetime of an individual and 

 the differentiation which the leg of some primseval mammal 

 must have undergone before it could have given rise to such a 

 strikingly peculiar limb as the foot of .the mole, there lies a 

 great gulf which no experience hitherto attainable enables 

 us to bridge over. The most we can say is this : That, beyond 

 a doubt, some cause unknown to us must exist — or must have 



ryJ 



Fig. 83. — ^A piece of wood bored by Zimnoria terebrans, from Heligoland. 



existed — in the nature of different animals, which has occasioned 

 them sometimes to abandon their original habits or to alter 

 them. 



Fig. 84. — A piece of solid limesi^onc bored by Limnoria terebrans, from Ireland. 



The two woodcuts here given illustrate a very striking case 

 in point. It has long been known that a small Crustacean, 

 Limnoria terebrans, attacks the hardest kinds of wood — like the 

 well-known ship-worm, Teredo navalis — and pierces it in all 

 directions with its cylindrical galleiies (see fig. 83). But it is 

 perhaps less well known that the same species attacks solid 

 limestone in the same manner. The stone of which the an- 

 nexed cut shows a small portion (see fig. 84) T myself picked up 



