PHYSICAL CONDITIONS OF THE DEEP SEA. 463 



and the tube had suddenly given way and been crushed by the 

 violence of the action to a fine powder. So violent and rapid had 

 been the collapse that the water had not had time to rush in by 

 means of the holes at both ends of the copper cylinder and thus 

 fill the empty space left behind by the collapse of the glass tube, 

 but had instead crushed in the copper wall and brought equilib- 

 rium in that manner. The process is exactly the reverse of an 

 explosion, and is termed by Sir Wy ville Thomson an ' implosion/ " 

 It is only reasonable to suppose that the ability to sustain this 

 enormous pressure can only be acquired by animals after genera- 

 tions of gradual migrations from shallow waters. Those forms 

 that are brought up by the dredge from the depths of the ocean 

 are usually killed and distorted by the enormous and rapid dimi- 

 nution of pressure in their journey to the surface, and it is ex- 

 tremely probable that shallow-water forms would be similarly 

 killed and crushed out of shape were they suddenly plunged into 

 very deep water. The fish that live at these enormous depths are, 

 in consequence of the enormous pressure, liable to a curious form 



Equator -* Pole 



30*1 Superficial 



zff-> waters 



Intermediate 

 waters 



f= / -^ 5* 



s - =y A ^^ 



5 _ / \ 4 Bottom 

 / \- 3" waters 



s 



-2' 



FIG. 1. DIAGRAM ILLUSTRATING THE PASSAGE OF AN OCEAN CURRENT ACROSS A BARRIER (A). 



of accident. If, in chasing their prey or for any other reason, 

 they rise to a considerable distance above the floor of the ocean, 

 the gases of their swimming bladder become considerably ex- 

 panded and their specific gravity very greatly reduced. Up to a 

 certain limit the muscles of their bodies can counteract the tend- 

 ency to float upward and enable the fish to regain its proper 

 sphere of life at the bottom ; but beyond that limit the muscles 

 are not strong enough to drive the body downward, and the fish, 

 becoming more and more distended as it goes, is gradually killed 

 on its long and involuntary journey to the surface of the sea. 

 The deep-sea fish, then, are exposed to a danger that no other 

 animals in this world are subject to namely, that of tumbling 

 upward. That such accidents do occasionally occur is evidenced 

 by the fact that some fish, which are now known to be true deep- 

 sea forms, were discovered dead and floating on the surface of the 

 ocean long before our modern investigations were commenced. 



Until quite recently, every one agreed that no rays of sunlight 

 could possibly penetrate the sea to a greater depth than a few 



