32.-, 



Landwards, the littoral district interlocks with the corre- 

 sponding districts of the terrestrial and fluvial realms, the 



faunas and floras of all more or less intermingling;. It is in 

 this portion of the littoral district that an important sub- 

 division must be considered, namely, the shore. The shore 

 is that part of the littoral district which lies between the 

 highest water mark (often considered as even including the 

 highest point of the wave mark) and the lowest line drawn 

 during the lowest ebb. In the greater part of this division 

 of the littoral district there is a change of medium twice 

 every twenty -four hours, and a change of the consequent 

 physical conditions attendant upon the character of the 

 medium. Organisms living in this portion of the littoral 

 zone must be capable of withstanding- the effects of the par- 

 tial or complete removal of their normal medium for a 

 greater or less time. It is here that the interlocking of the 

 marine and terrestrial faunas and floras becomes most 

 marked, and an intermingling, and a migration from one dis- 

 trict to the other, occurs. Migration from the land to the sea 

 is seen in the whales, seals, and other aquatic mammals, 

 which have become marine in so far as their mode of locomo- 

 tion is concerned. Owing, however, to the inability of air- 

 breathing animals to adapt themselves to a water-breathing 

 habit, all terrestrial animals passing into the sea must 

 assume a pelagic life, where they can retain their normal 

 method of breathing. 



Among other animals which have exchanged their normal 

 terrestrial habit for a prevailingly marine one, may be men- 

 tioned, several birds, such as the Penguin and the Albatross, 

 certain snakes, turtles, and crocodiles, and a number of 

 insects. The birds and insects here considered represent a 

 passage from the aerial to the marine pelagic; while the rep- 

 tiles, like the mammals, illustrate a passage from the land to 

 the pelagic district of the sea. 



While thus the land fauna, in advancing into the sea, nat- 

 urally takes to a pelagic life, the land flora can adapt itself to 

 the conditions of the littoral district. This is well shown bv 



