16 C. HEDLEY. 



i 



mud, and from shelter to exposure are here concentrated 

 in an area conveniently small for comparative study. The 

 shore line of the Port Jackson "ria" winds by so many 

 creeks and coves that from Head to Head it extends for 

 one hundred and eighty-three miles. 



The intertidal zone around Sydney may be grouped, 

 ecologically, into three divisions, decided by the situation 

 of the fauna and flora; viz., (1), the sandy beach, (2), the 

 muddy estuary, and (3), the rocky reef. Each of these 

 areas maintains a marine community which by internal 

 relation and external distinction rank with such a ter- 

 restrial society as may dwell in a forest, a marsh or a 

 meadow. These three associations of beach life are 

 repeated, with slight local modifications, over such wide 

 geographic space, perhaps even right round the world, that 

 we may regard them as having persisted with little change 

 for a long geologic period. 



As the plants of an English forest may be more closely 

 related to those of an American forest in another hemi- 

 sphere than to those of a fen in a neighbouring county, so 

 the marine fauna of the Parramatta River is more akin to 

 another estuarine fauna, even if it be one or two thousand 

 miles away, than to the rock fauna of the nearby Bondi 

 Beach. It is as if two cities stood side by side, yet remained 

 foreign to each other in race, language, and customs. 



Though the ocean does not present to migration such 

 barriers, — mountains, deserts or forests, — as does the land, 

 marine species rarely, but genera frequently, attain a 

 world wide range. 1 The sea being more monotonous than 

 the land would less frequently originate new types. But 

 when it did come into existence a new marine type would 

 spread more readily than a terrestrial one, hence the greater 

 uniformity of marine life. In the sea as well as upon the 



1 M'Intosh, Ann. Mag. Nat. Hist. (7), xiii, 1904, p. 130. 



