Adaptations to Environment 123 



shore and the shallow water. After undergoing radical changes 

 and gaining strength, the young creatures return to the shore in 

 various ways. 



III. THE DEEP SEA 



Very different from all the other haunts are the depths of the 

 sea, including the floor of the abysses and the zones of water near 

 the bottom. This haunt, forever unseen, occupies more than a 

 third of the earth's surface, and it is thickly peopled. It came 

 into emphatic notice in connection with the mending of telegraph 

 cables, but the results of the Challenge?' expedition (1873-6) 

 gave the first impressive picture of what was practically a new 

 world. 



Physical Conditions 



The average depth of the ocean is about two and a half miles ; 

 therefore, since many parts are relatively shallow, there must be 

 enormous depths. A few of these, technically called "deeps," are 

 about six miles deep, in which Mount Everest would be engulfed. 

 There is enormous pressure in such depths ; even at 2,500 fathoms 

 it is two and a half tons on the square inch. The temperature is 

 on and off the freezing-point of fresh water (28-34 Fahr.), due 

 to the continual sinking down of cold water from the Poles, 

 especially from the South. Apart from the fitful gleams of 

 luminescent animals, there is utter darkness in the deep waters. 

 The rays of sunlight are practically extinguished at 250 fathoms, 

 though very sensitive bromogelatine plates exposed at 500 

 fathoms have shown faint indications even at that depth. It is 

 a world of absolute calm and silence, and there is no scenery on 

 the floor. A deep, cold, dark, silent, monotonous world ! 



Biological Conditions 



While some parts of the floor of the abysses are more thickly 

 peopled than others, there is no depth limit to the distribution of 



