116 New York State Museum 



quieter waters over the continental shelf. Especially 

 in estuaries plants are an important factor in building 

 up mud flats until they become firm ground above the 

 influence of the sea. There is a tendency toward the 

 formation of tide channels in weak spots, and mussels 

 line the muddy banks of such channels to such an ex- 

 tent that erosion is almost stopped. 



The nature of the shore at a certain spot plus the 

 other physical conditions with which organisms must 

 contend determines the character of the plant and 

 animal associations. If there is a uniformity of physi- 

 cal conditions over considerable areas and distances 

 there will be a most pronounced and well-marked 

 fauna, but also a more limited one than where there 

 is a diversity of conditions. If the substratum in two 

 places is the same, then any difference in the flora and 

 fauna is due to difference in exposure. Some forms 

 can not stand much exposure, others require a certain 

 amount of exposure. If a sandy beach succeeds rocks 

 along the shore there is a change in the type of life, 

 but when the same type of shore appears again, the 

 same plant and animal associations reappear. There 

 is also this to be remembered, whether along the At- 

 lantic, Pacific or other coast, whether in tropical, 

 temperate or arctic regions, the same type of shore 

 under the same physical conditions (exposure to winds, 

 tides etc.) will show the same character in the plant 

 and animal associations. The species, even the genera 

 may be different, but the types are the same. In an 

 association a normal relation between the various 

 members has come to exist. There are a certain num- 

 ber of genera, species and individuals which may be 

 looked for in an association. Where there are several 



