INTRODUCTORY. 11 



we took living, well marked, and characteristic 

 examples of all the five invertebrate sub-kingdoms. 

 And thus the question of the existence of abundant 

 animal life at the bottom of the sea has been finally 

 settled, and for all depths, for there is no reason to 

 suppose that the depth anywhere exceeds between 

 three and four thousand fathoms ; and if there be 

 nothing in the conditions of a depth of 2,500 fathoms 

 to prevent the full development of a varied fauna, it 

 is impossible to suppose that even an additional 

 thousand fathoms would make any great difference." 



"The conditions which might be expected prin- 

 cipally to affect animal life at great depths of the 

 sea are pressure, temperature, and the absence of 

 light, which, apparently, involves the absence of 

 vegetable food. Beyond the zone surrounding the 

 land, speaking generally, the average depth of the 

 sea is 2,000 fathoms, or about two miles ; as far 

 below the surface as the average height of the Swiss 

 Alps. In some places the depth seems to be con- 

 siderably greater, possibly here and there nearly 

 double that amount ; but these abysses are certainly 

 very local, and their existence is even uncertain, and 

 a vast portion of the area does not reach a depth of 

 1,500 fathoms." 



Having given these general estimates of the depth 

 of the ocean, Professor Thomson deals with the three 

 supposed conditions inimical to animal life. And 



