418 THE INHABITANTS OF THE SEA. 
sufficient to put any idea of life out of the question? There 
was a curious popular notion that on descending deeper and 
deeper the sea water became gradually, under the pressure, 
heavier and heavier, so that at last it became more weighty than - 
molten gold. But water is, in fact, almost incompressible; so 
that its density at 2,000 fathoms is scarcely appreciably in- 
creased. Any free air suspended in the water, or contained in 
any compressible tissue of an animal at 2,000 fathoms, would 
of course be reduced to a mere fraction of its bulk; but the 
animals subject to the pressure of the deep seas, being permeated 
throughout their whole organisation by incompressible fluids at 
the same pressure, are consequently as capable of bearing it as 
we do the pressure of the atmosphere. The absence of light 
seemed another circumstance incompatible with the existence 
of animal life at abyssal depths, as all plants depend upon 
light for their growth, and their absence apparently involves 
that of vegetable food, which, as we all know, forms everywhere 
the substratum of animal existence. 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 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; and 
though probably some portions of the sun’s light possessing 
certain properties may penetrate to a much greater distance, it 
is certain that, beyond the first fifty fathoms, plants to whose 
existence light is essential are barely represented, and after two 
hundred fathoms entirely absent. 
But though plant-life is thus limited to the more superficial 
parts of the ocean, the analysis of sea water, taken in all 
localities and at all depths, has shown that it everywhere 
contains a very appreciable and very uniform quantity of 
organie matter in solution and in suspension. it is thus 
quite intelligible that numberless protozoa—whose distinctive 
character is that they are capable of being supported by the 
absorption of organic matter through the surface of their 
bodies—are able to exist in the dark abysses of the sea, and in 
their turn afford nourishment to more highly organised animals. 
After these general remarks on the creatures of the deep, 
I will now give a brief account of their various groups. 
