28 THE FAUNA OF THE DEEP SEA 



probably the most important for them in searching for 

 their food. That is quite probable ; but many shallow- 

 water animals invariably seek their food by their 

 sense of smell without showing any traces of a weak- 

 ness in their &ense of sight. It m-iy be taken as an 

 axiom of biology that unless a particular sense is 

 absolutely useless to an animal or a positive disad- 

 vantage to it, that sense will be retained. 



It may be stated then with some confidence that 

 in the abysmal depths of the ocean there is no trace 

 of sunlight. It is highly improbable, on the face of 

 it, that any ray of light could penetrate through a 

 stratum of water four miles in thickness, even if the 

 water were perfectly pure and clear, but when we 

 remember that the upper regions, at least, are crowded 

 with pelagic organisms provided with skeletons of 

 lime and si-lica, we may justly consider that it is im- 

 possible. 



The temperature of the water in the abyss is by 

 no means constant for a constant depth nor does it 

 vary with the latitude. It is true that, as a rule, the 

 water is colder at greater depths than in shallower ones, 

 and that the deeper the thermometer is lowered into 

 the sea, the lower the mercury sinks. This is consist- 

 ent with physical laws. If there is any difference at 



