PLANT AND ANIMAL ASSOCIATIONS 41 



the specialised, upper portion of the Littoral which encircles 

 the land margin and is usually considered to end at a depth 

 of 100 fathoms. The extent of the Littoral varies with 

 the structure of the region under consideration — the British 

 Isles, for instance, are situated on a submarine plateau which 

 forms part of the European " Continental Shelf," and the 

 lOO-fathom Hmit will be found to include a portion of the 

 Atlantic to the west of Ireland and Scotland ; the Irish 

 Sea ; the English Channel and the North Sea. Much of 

 the North Sea has a depth of less than 300 feet and, except 

 in a few places, the whole is less than 600 feet. 



Algal Associations. — As has already been indicated, 

 the interaction of the numerous physical peculiarities of the 

 tidal region has resulted in a sorting out of the shore popula- 

 tion into series capable of inhabiting the successive horizons 

 or zones that occur between high and low tide marks. This 

 *' zoning " is one of the first facts that strikes an intelligent 

 observer. The algal life is affected, and through it the 

 animal life. Where conditions vary, zones will vary. Par- 

 ticular strata spread out when conditions are favourable, and 

 " pinch out " when the struggle becomes too severe 

 (Pearse, 19 14). 



Marine vegetation falls into two great subdivisions : 



I. Free-floating or free-swimming plants : Plankton 

 (haloplankton). 



II. Fixed plants : Benthos (Warming, 1896). 



It must be noted that, in contrast to the vegetation on 

 land, the fixed vegetation of the sea is greatly localised 

 (Petersen and Jensen, 191 1). Cotton (19 12) distinguishes 

 three main regions : 



1 . The Littoral region, extending from the level of highest 

 marine vegetation to that of low- water mark, neap tide. 



2. The Sub-littoral, which begins from the lower limit 

 of the last named, and extends down to the limit of seaweeds 

 (about 25 fathoms). 



3. The EUttoral, which is devoid of vegetation. 



He further recognises three principal formations (for the 

 west of Ireland), {a) rocky shore formations, {h) sand and 



