325 



Landwards, the littoral district interlocks with the corre- 

 sponding districts of the terrestrial and tlnvial realms, the 

 fannas and floras of all more or less interminglino-. It is in 

 this portion of the littoral district that an im])ortant 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 ])oint of the wave mark) and the lowest line di-awn 

 during the lowest ebb. In the greater ])art of tliis division 

 of the littoral district there is a change of medium twice 

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

 physical conditions attenchint u])on 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 remf)val 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 ac[uatic 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 ])assing into the sea must 

 assume a pelagic life, where they can retain their nornml 

 method of l)reathing. 



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; Avhile 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 w^ell shown bv 



