298 FISHES. 



2. The absence of sunlight is in some measure compen- 

 sated for by the presence of phosphorescent light, produced by 

 many marine animals, and also by numerous Deep-sea fishes. 



3. Depression and equality of the temperature. At a depth 

 of 500 fathoms the temperature of the water is already as 

 low as 40° Fahr., and perfectly independent of the temperature 

 of the surface-water ; and from the greatest depths upwards to 

 about 1000 fathoms the temperature is uniformly but a few 

 degrees above freezing-point. Temperature, therefore, ceases to 

 offer an obstacle to the unlimited dispersal of Deep-sea fishes. 



4. The increased pressure by the water. The pressure of 

 the atmosphere on the level of the sea amounts to fifteen 

 pounds per square mch of the surface of the body of an 

 animal ; but the pressure amounts to a ton weight for every 

 1000 fathoms of depth. 



5. With the sunlight, vegetable life ceases in the depths of 

 the sea. All Deep-sea fishes are therefore carnivorous ; the 

 most voracious feeding frequently on their own offspring, and 

 the toothless kinds being nourished by the animalcules which 

 live on the bottom, or which, " like a constant rain," settle 

 down from the upper strata towards the bottom of the sea. 



6. The perfect quiet of the water at great depths. The 

 agitation of the water, caused by the disturbances of the air, 

 does not extend beyond the depth of a few fathoms ; below 

 this surface-stratum there is no other movement except the 

 quiet flow of ocean-currents, and near the bottom of the 

 deep sea the water is probably in a state of almost entire 

 quiescence. 



The effect upon fishes of the physical conditions described 

 is clearly testified by the modification of one or more parts of 

 their organisation, so that every Deep-sea fish may be recog- 

 nised as such, without the accompanying positive evidence 

 that it has been caught at a great depth; and vice versa, 

 fishes reputed to have been obtained at a great depth, and not 



