CHAP. I.] INTRODUCTION. 45 



The fauna becomes more uniform over a larger area, 

 and is manifestly one of wliicli tlie shallower water 

 fauna of some colder region is to a great extent a 

 lateral extension. Groing still deeper, the severity of 

 the cold increases until we reach the vast undulating 

 plains and valleys at the bottom of the sea, with their 

 fauna partly peculiar and partly polar — a region the 

 extension of whose extreme thermal conditions only 

 approaches the surface within the arctic and antarctic 

 circles. 



We have as yet very little exact knowledge as to 

 the distance to which the sun's light penetrates into 

 the water of the sea. According to some recent 

 experiments which will be referred to in a future 

 chapter, it would appear that the rays capable of 

 affecting a delicate photographic film are very rapidly 

 cut off, their effect being imperceptible at the depth 

 of only a few fathoms. It is probable that some 

 portions of the sun's light possessing certain pro- 

 perties 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 represented, and after 200 fathoms 

 they are entirely absent. The question of the mode 

 of nutrition of animals at great depths becomes, there- 

 fore, a very singular one. The practical distinction 

 between plants and animals is, that plants prepare 

 the food of animals by decomposing certain inorganic 

 substances which animals cannot use as food, and 

 recombining their elements into organic compounds 



