CHAP, i.] INTRODUCTION. 31 



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 any 

 where 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 develop 

 ment 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 principally 

 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. 



After passing a zone surrounding the land, which is 

 everywhere narrow compared with the extent of the 

 ocean, through which the bottom more or less abruptly 

 shelves downwards and the water deepens ; speaking 

 very 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 considerably 

 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 por 

 tion of the area does not reach a depth of 1,500 

 fathoms. 



The enormous pressure at these great depths seemed 

 at first sight alone sufficient to put any idea of life 

 out of the question. There was a curious popular 

 notion, in which I well remember sharing when a boy, 

 that, in going down, the sea- water became gradually 

 under the pressure heavier and heavier, and that all 

 the loose things in the sea floated at different levels. 



