37 



there. In a period of long continued calm, or in a whole 

 year of mild weather, the local influence may be notable. 

 However, in most years there must be long periods in which 

 the local influence is relatively negligible in determining 

 the condition of the populations. Doubtless, many of the 

 striking peculiarities of occurrence and abundance in 

 different seasons and years are due to differences in degrees 

 of dominance of local influences. The only reliable conclusion 

 possible seems to be that the coastal waters may or may not 

 exert noticeable influence upon the population within its 

 limits, depending on the extent to which water movements 

 permit its conditions to operate before the populations shift 

 to other localities or disappear. 



In the sea, a dense population of a particular phytoplank- 

 ton may appear in a locality for a few hours or days, and not 

 be strongly represented again for years. A plant or population 

 may appear at a level at which it is not usually found, or it 

 may be found most often at a certain level in one locality and 

 at a different level in another. Populations nearshore and at 

 considerable distances offshore may be closely similar, although 

 recognizable differences are more likely to be found. Inasmuch 

 as the shallow depth of water is sufficient to constitute a 

 fundamental difference from any offshore habitat, it is probable 

 that the presence of similar populations in places more or less 

 distant from each other is not necessarily an indication that 

 habitat conditions are also similar. Certain circumstances 

 may be responsible for such an irregularity. Particular environ- 

 mental features may be so prominent over a wide area at a given 



