CHAPTER VIII 
THE DISTRIBUTION OF ANIMAL LIFE 
IN THE SEA 
WHILE, as we have seen in the previous chapters, 
the natural regions of the land more or less grade 
into one another—savana, for example, passing into 
savana wood, and savana wood into forest, it is very 
much easier to separate the oceans into natural regions, 
for here the divisions are relatively sharp. The two 
great conditions which influence the type of animal to 
be found within any part of the sea are, first, the pres- 
ence or absence of light, and, second, the presence or 
absence of asubstratum. At the margin of the sea, where 
the light penetrates to the bottom, we have the littoral 
area, peopled by littoral organisms. All these organisms 
agree in that, in structure or in habit, they show adapta- 
tions to the two striking features of the area. Thus 
a sea anemone, fixed to a rock and containing symbiotic 
algae, is obviously adapted for life in a region where 
a substratum is present, and where light penetrates. 
A plaice, so shaped that it is adapted for lying on 
a sandy surface and bearing well-developed eyes, is 
similarly fitted. But the dependence of a mud-haunt- 
ing animal feeding upon minute algae upon littoral 
conditions is no less real, if less apparent, for land- 
derived mud only accumulates off shores, and the 
minute algae must have light before they can live. 
In the open water occurs another group of organisms, 
