329 



sea, where light and s^hade, a genial temperature, currents 

 changeable in power and dii-ection, 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 pla}' the tendencies to varv 

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

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

 nal conditions." This district may perhaps be regarded as 

 the cradle oi 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, oi' 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 \^ith the cori'esponding 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 temporar}^ existence in the pelagic 

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

 bottom or slnjres for food. Occasionally, inhabitants of 

 the pelagic district enter for a time the corres|)()nding dis- 

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

 tlie 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 tyi)es 

 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 (|uite probable, as Sir William 

 Flower suggests, that theCetacea, 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 



