CHAP. I.] INTRODUCTION. 31 
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 principally 
to affect animal life at great depths of the sea are 
pressure, temperature, and the absence of ight 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, anda vast 
portion 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, 
