INFLUENCE OF CLIMATE. 409 
phical 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 desert tracts of unprolific sand, which in many places are 
spread out in extensive shallows, are all-powerful influences, 
determining their diffusion within certain and more or less defined 
limits.” 
The structure of the coast, as far as the mineral character of 
its rocks is concerned, may seriously affect the distribution of 
particular tribes. Since many shell-fish, for instance, bore only in 
limestone or rocks containing abundance of lime, a very ordinary 
difference in the nature of the strata must necessarily determine 
their presence or absence. 
The outline of a coast has also great influence in regulating 
the diffusion of species. A much indented region is very 
favourable to submarine life; a straight coast-line, exposed to the 
full rolling of the surf, is 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, as Mr. Godwin 
Austen quaintly remarks, “ of those sturdy people, not uncommon 
in this stormy life, who thrive best in troubles, and feel happiest 
under conditions that 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, to a great extent, the presence or absence of 
peculiar forms of shell-fish and other invertebrata, and of fish 
also, since the distribution of the food regulates that of the 
devourers. 
The rise and full of the tides are most important in deter- 
mining the presence or absence of the species inhabiting the 
littoral zone. The currents, besides their agency as modifiers 
of climate, act as means of transport, by carrying the germs and 
larvae of numerous creatures from region to region. 
The influence of climate is conspicuously manifested in the 
diminution of the number of genera and species as we proceed 
northwards to the Iey Ocean. 
