i] THE CATEGORIES OF LIFE 13 



would have disappeared and a delicate photographic 

 plate exposed here for over two hours would not be 

 affected. Absolute darkness exceeding that of the 

 photographer's dark room would prevail. 



We can easily imagine the cold, and the darkness^"? 

 and the great plains of semi-liquid mud, but not the 

 extreme pressure of the water. At the surface of 

 the sea an organism is exposed to a pressure of one 

 atmosphere, that is 15 pounds per square inch of its 

 surface, and a rise of even one atmosphere produces 

 very unpleasant effects in ourselves. But for every 

 ten metres (5 fathoms) that we descend, the pressure 

 due to the weight of the water rises by one atmo- 

 sphere, and at a depth of 3000 fathoms the pressure 

 on every square inch of the body of an animal 

 amounts to about three tons. If we did not kndw 

 that no part of the sea-bottom however deep is 

 utterly devoid of animal life we might conclude as 

 the older naturalists did that this enormous pressure 

 was inconsistent with a belief in the existence of 

 living organisms. Even now it is difficult to under- 

 stand how the tissues of an animal are able to 

 withstand it. As it is, the pressure exerts a profound 

 influence on the structure of the animals inhabiting 

 even such moderate depths as 200 fathoms. A deep- 

 sea fish is easily recognised by its large eyes, its long 

 and attenuated tail, and the softness of its flesh. 

 One's finger leaves a dent in the flesh of such a fish 



