IN TROD UCTOR Y. 



only approaches the surface within the arctic and 

 antarctic circles." 



Finally, as to the absence of light. Very little 

 exact knowledge on this point has been obtained. 

 From recent experiments " it would appear that the 

 rays capable of affecting a delicate photographic film 

 are very rapidly cut off, their effect being imper- 

 ceptible at the depth of only a few fathoms. It is 

 probable that some portions of the sun's light pos- 

 sessing certain properties may penetrate to a much 

 greater distance, but it must be remembered that 

 even the clearest sea-water is more or less tinted by 

 suspended opaque particles, and floating organisms, 

 so that the light has more than a pure saline solution 

 to contend with. At all events it is certain that 

 beyond the first 50 fathoms, plants are barely repre- 

 sented, and after 200 fathoms they are entirely 

 absent. There seems to be little or no light at the 

 bottom of the sea, and there are certainly no plants 

 except such as may sink from the surface, but the 

 bottom of the sea is a mass of animal life." 1 



It has been seen that pressure presents no obstacle 

 to animal life at great depths, that temperature is 

 not carried below that at which animal life may be 

 sustained, but that there is practically an absence of 

 light, and consequently of vegetable life. 



1 " The Depths of the Sea." By C. Wyville Thomson. 

 Introductory Chapter, pp. 1-48. 



