18 



THE NATURAL HISTORY OF 



through a sudden change in the structure of the 

 land and sea-bed. The diffusion of burrowing 

 marine invertebrata must be very seriously affected 

 by such changes. Thus many shell-fish bore only 

 in limestones, or rocks containing abundance of 

 lime ; a very ordinary difference in the nature of the 

 coast must determine their presence or absence. 



The outline of a coast has great influence in regu- 

 lating the diffusion of species. A much indented 

 region is very favourable to submarine life, a straight 

 and exposed coast-line usually unfavourable, though 

 there are a few creatures which delight in the dash 

 of the waves, and hardily — though some of them 

 are small and exceedingly delicate — brave the full 

 force of the ocean-storms ; reminding us of those 

 sturdy people not uncommon in this stormy life, who 

 thrive best in troubles, and feel happiest under con- 

 ditions that make most men miserable. 



The nature of the sea-bottom determines, to a great 

 extent, the presence or absence of peculiar forms of 

 shell-fish and other invertebrata, and of fish also, 

 since, according to the food, so is the distribution of 

 the devourers. We find very different creatures 

 brought up by the line, net, or dredge, according as 

 the bottom is of mud, sand, gravel, nullipore (coral- 

 like sea-weed), broken shells, loose stones or rock, 

 and the gradations of their intermixtures. 



Tides are also modifying influences, and the ex- 

 tent to which they rise and fall is most important in 

 determining the presence or absence of the species 



