THE LITTORAL DISTRICT 987 



with Fuciis vesiculosus and .IscopJiylluin nodQsurn predominating. 

 At low-tide these hang Hke a wet fringe over the exposed rocks 

 and give shelter to numerous species of the smaller littoral animals, 

 as well as other algae. 



The littoral districts of the marine and fluvial realms also inter- 

 lock along the shores where streams mouth into the sea or expand 

 into broad estuaries. Here marine animals will venture up into the 

 fresh water littoral district, while, similarly, fresh water animals 

 pass into the littoral district of the marine realms. The common 

 meeting-ground of the two approaching floras and faunas is in 

 the estuarine or brackish water facies of the littoral districts. 



The neritic zone, "Flachsee." or thalassic zone, is that portion 

 of the littoral district which is never uncovered. It is separated 

 as a distinct district by Walther, Haug, and others, who restrict 

 the term littoral to the shore zone. It is, however, so intimately 

 connected with the shore zone in all its physical and bionomic 

 characteristics that a separation is not natural. The bottom of 

 the neritic zone of the littoral district is less diversified than that 

 of the shore zone. In its upper portion and. in its shoals it may 

 partake of the character of the shore zone, but in its deeper por- 

 tions the character of the bottom is usually more uniform, being 

 either rocky or, what is more common, composed of fine detrital 

 material mingled with organic matter in various stages of dissolu- 

 tion. According to the character of the bottom, plant life will vary 

 and with it, to a greater or less extent, animal life. 



Taken as a whole, the littoral district is the most important 

 portion of the sea, both from a bionomic point of view and from its 

 bearing on palaeontology. "The littoral region," says Loven (4:5'(5). 

 "comprises the favored zones of the sea. where light and shade, a 

 genial temperature, currents changeable in power and direction, 

 a rich vegetation spread over extensive areas, abundance of food, of 

 prey to allure, of enemies to withstand or evade, represent an in- 

 finitude of agents competent to call into play the tendencies to vary 

 which are embodied in each species, and always ready, by modify- 

 ing its parts, to respond to the influences of external conditions." 

 This district may perhaps be regarded as the cradle of organic 

 life, from which were peopled the abyssal and pelagic districts, on 

 the one hand, and the terrestrial and fluvial realms and their vari- 

 ous districts, on the other. 



B. Fresh Water. The littoral district of the fresh water realm 

 is almost coextensive in area with that of the streams and fresh 

 water lakes and ponds of the world. The only exception to this is 

 found in those portions of very deep lakes where sunlight does not 



