INFLUENCE OF CLIMATE. 409 



pLical assemblages of their terrestrial brethren ; and such is to a 

 great extent true. Nevertheless, the inequalities of the sea-bed, 

 the modifications of the temperature of the ocean produced by 

 currents pouring through it like mighty rivers, the projection of 

 promontories, and the more important interruptions caused by 

 the great gulfs and abysses of the deep, or by vast and compara- 

 tively cesert tracts of unprolific sand, which in many places are 

 spread iut in extensive shallows, are all-powerful influences, 

 determining their diffusion within certain and more or less defined 

 limits." 



The stricture of the coast, as far as the mineral character of 

 its rocks ii concerned, may seriously affect the distribution of 

 particular tabes. Since many shell-fish, for instance, bore only in 

 limestone or rocks containing abundance of lime, a very ordinary 

 difference in <:he nature of the strata must necessarily determine 

 their presenct or absence. 



The outlmeof a coast has also great influence in regulating 

 the diffusion ->f species. A much indented region is very 

 favourable to sibmarine life ; a straight coast-line, exposed to tha 

 full rolling of tie surf, is usually unfavourable, though there are a 

 few creatures whch delight in the dash of the waves, and hardily, 

 though some of them are small and exceedingly delicate, brave 

 the full force of he ocean storms, reminding us, as Mr. Godwin 

 Austen quaintly renarks, " of those sturdy people, not uncommon 

 in this stormy life, who thrive best in troubles, and feel happiest 

 under conditions tiat make most men miserable." 



The nature of the sea-bottom, according as it consists of 

 mud, sand, gravel nullipore, broken shells, loose stones, or 

 rock, determines, tc a great extent, the presence or absence of 

 peculiar forms of s^ell-fish and other invertebrata, and of fish 

 also, since the distribution of the food regulates that of the 

 devourers. 



The rise and fall of the tides are most important in deter- 

 mining the presence or absence of the species inhabiting the 

 littoral zone. The cvrrents, besides their agency as modifiers 

 of climate, act as meais of transport, by carrying the germs and 

 larvss of numerous creiures from region to region. 



The influence of chnate is conspicuously manifested in the 

 diminution of the numler of genera and species as we proceed 

 northwards to the Icy Geaii. 



