22 THE FAUNA OF THE DEEP SEA 



to a considerable distance above the floor of the 

 ocean, the gases of their swimming bladder become 

 considerably expanded and their specific gravity 

 very greatly reduced. . Up to a certain limit the 

 muscles of their bodies can counteract the tendency 

 to float upwards 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 downwards, 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 upwards. 



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 hundred fathoms. 



Moseley says that ' probably all is dark below 

 200 fathoms excepting in so far as light is given out 

 by phosphorescent animals,' and Wyville Thomson 



