﻿226 Proceedings of the Royal Physical Society. 



of such typical faunas and the correlation of the differences 

 they exhibit with the peculiarities of the environment of 

 each. It would be found that the fauna of each distinct 

 region consisted of a typical constellation of species, such 

 species being present as were adapted to the particular 

 conditions. Burrowing animals are found on sandy and 

 muddy shores, sedentary animals on fixed rocks and stones, 

 and so on. But it might also be found that certain species 

 were common to two or more of these regions, but that 

 they appeared to have undergone typical modification. We 

 might distinguish then, Association differences, or local 

 differences in the grouping of species, from Modification 

 differences, or local differences in individuals of a species. 

 To appreciate these differences it would be necessary to 

 make an intensive study of a sample of each type of shore. 

 Suppose we consider then a definite area on the shore 

 belonging to one of these distinct kinds of shore. It is 

 clear that even within such an area we shall find local 

 variations in the fauna. A very slight acquaintance with 

 shore life is sufficient to show one that the fauna at or near 

 low-water mark is not only richer than, but different in 

 character and composition from the fauna at higher levels. 

 For convenience of study and to make our observations 

 systematic, it will be necessary to divide up the beach into 

 Zones according to depth. 



Another circumstance which we can reasonably suppose 

 to have considerable influence upon the character of the 

 shore fauna is the occurrence of fresh-water streams flowing 

 down the beach. It will be necessary, then, to mark off the 

 area traversed by the fresh water, and compare its inhabitants 

 with those of the neighbouring parts of the shore. 



It is a fact known to all workers who have had experience 

 in shore collecting, that many animals are to be found 

 habitually in certain positions on the shore, lurking under 

 stones, attached to the under surface of loose boulders, ad- 

 hering to weed, and so on. It will be necessary, then, to dis- 

 tinguish Positions of various kinds and compare the typical 

 faunas of these. It seems to us that only by going into 

 details of this kind do we get into touch with the real 



