THE LITTORAL DISTRICT 985 



carrying a large amount of debris into tiie sea and thus produce 

 conditions unfavorable to the existence of many organisms. 



Landward the littoral district interlocks with the corre- 

 sponding districts of the terrestrial and t^uvial realms, the faunas 

 and floras of all more or less intermingling. It is in this portion 

 of the littoral district that an important subdivision must be con- 

 sidered; namely, the shore. The shore, as already noted (ante, 

 Chapter XV), is that part of the littoral district which lies between 

 the highest water mark (often considered as including even 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 dis- 

 trict 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 partial or complete removal of their normal medium for a 

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

 trial and marine floras and faunas becomes most marked, and an 

 intermingling and a migration from one district into the other 

 occur. Migration from the land to the sea is exemplified by the 

 whales, seals, and other aquatic mammals, which have become 

 marine in so far as their mode of locomotion is concerned. Owing 

 however, to the inability of air-breathing animals to adapt them- 

 selves 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 respiration. 



Among other animals which have exchanged their normal ter- 

 restrial habit for a prevailingly marine one may be mentioned 

 several birds, such as the Penguins 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 district ; while the reptiles, 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, naturally 

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

 tions of the littoral district. This is well shown by the eight spe- 

 cies of phanerogams, which have acquired a wholly marine habit, 

 and are now known as eel grasses or sea grasses. In the case of 

 these plants the adaptation is so complete that they can no longer 

 live out of their adopted habitat. The mangrove plants, on the 

 other hand, are only partially accommodated to the conditions of 

 the marine littoral district, for it is necessary that their crowns of 

 leaves should be above water. 



