STUDIES IN MARINE ECOLOGY. 245 



in such studies as the present than is the distribution of oxygen. 



Both factors, however, may range widely within a given 

 association. Take for example the sensitive Amar cerium associa- 

 tion which flourishes on wharf pilings or on exposed rocks. At 

 the mouth of Northwest Gutter, where the pH stays about 8.0 

 and the oxygen ranges from 3.72 to 4.95 c.c. per L., the rocks 

 support a typical rock- Amarcecium association. Yet the same' 

 association, fully as rich in species, occurs at Gansett on similar 

 rocks, but with much plant life- all about and with the pH varying 

 from 8.0 to 8.3 and the oxygen from 5.15 (and probably lower) 

 to 8.56 c.c. per L. It will be noted that the range of conditions 

 either at Northwest Gutter or at Gansett is lower than when the 

 two are combined, so that, while the association can exist in 

 these limits, it is not subjected to these extremes in one locality. 

 The total range in both temperature and salinity is likewise 

 greater when the two locations are considered together than when 

 either is taken singly. 



It is probable that a collection of data concerning the conditions 

 under which the different sensitive animals live in all their 

 different localities would, when thrown together, indicate that 

 they could stand widely differing concentrations of all the water 

 factors considered; when, as a matter of fact, they are exposed 

 to relatively slight changes in the location in which they do live. 



With less sensitive animals the association limits as set out in 

 this paper mean nothing. Pagurus longicarpus apparently roams 

 at will in all of those of the flats and occurs among those of the 

 rocks, carrying with him, willy-nilly, his commensals. The mud 

 snail, Nassa obsoleta, is at present found among all the associa- 

 tions, from the clean sand to the inter-tidal associations, and, 

 according to Dimon, originally dominated the sand also before 

 being driven off by Littorina litorea. In part it is able to do this 

 because it is a resistant animal and in part on account of the fact 

 that it probably becomes accustomed to conditions in a given 

 locality and tends to keep within them. In this regard, one can 

 but express the wish for more studies like that of Dimon. With 

 a series, of such studies at hand one could draw definite conclu- 

 sions where now, so far as individuals are concerned, he is limited 

 in large part to theorizing. 



There is so much work necessary in making an ecological 



