321) 



sea, whore 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 infinitude of 

 agents competent to call into play the tendencies to vary 

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

 modifying its parts, to respond to the influences of exter- 

 nal conditions." This district may perhaps be regarded as 

 the cradle of organic life, from which, on the one hand, 

 were peopled the abyssal and pelagic districts, and, on the 

 other, the terrestrial and fluvial realms and their various 

 districts. 



The marine pelagic, or halo-pelagic district, or more 

 briefly, the pelagic district, is the common meeting-ground 

 of most of the life districts. It touches all shores and com- 

 municates with the corresponding districts of both terres- 

 trial and fluvial realms. It has direct communication with 

 the littoral district, many inhabitants of which leave the 

 bottom at times to lead a temporary existence in the pelagic 

 district; while, in turn, many pelagic animals visit the 

 bottom or shores for food. Occasionally, inhabitants of 

 the pelagic district enter for a time the corresponding dis- 

 trict of the terrestrial realm, i. e., the aerial, as, for example, 

 the so-called flying fish ; and, in turn, as already noted, many 

 aerial animals spend a, part of their lives in the marine 

 pelagic district, or, at least, show a decided preference for a 

 pelagic life. The passage of land animals to the halo-pelagic 

 district has already been noted. Similar intermingling of 

 fresh water, or limno-pelagic, and salt water pelagic types 

 occurs in the estuaries and stream mouths, and it is well 

 known that halo-pelagic fish will enter the limno-pelagic dis- 

 trict in breeding time. It is quite probable, as Sir William 

 Flower suggests, that the Cetacea, in their transition from 

 a terrestrial to a marine life, passed through a stage in 

 which they lived in fresh water. A similar transition for 

 the sea-grasses is not improbable, though they can no 

 longer live in fresh water. Intercommunication between the 



