CHAP. 1.] INTRODUCTION. 45 
The fauna becomes more uniform over a larger area, 
and is manifestly one of which the shallower water 
fauna of some colder region is to a great extent a 
lateral extension. Going still deeper, the severity of 
the cold increases until we reach the vast undulating 
plains and valleys at the bottom of the sea, with their 
fauna partly peculiar and partly polar—a region the 
extension of whose extreme thermal conditions only 
approaches the surface within the arctic and antarctic 
circles. 
We have as yet very little exact knowledge as to 
the distance to which the sun’s hight penetrates into 
the water of the sea. According to some recent 
experiments whieh will be referred to in a future 
chapter, 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. It is probable that some 
portions of the sun’s light possessing certain pro- 
perties may penetrate to a much greater distance, but 
it must be remembered that even the clearest sea-water 
is more or less tinted by suspended opaque particles 
and floating organisms, so that the light has more 
than a pure saline solution to contend with. At all 
events it is certain that beyond the first 50 fathoms 
plants are barely represented, and after 200 fathoms 
they are entirely absent. The question of the mode 
of nutrition of animals at great depths becomes, there- 
fore, a very singular one. The practical distinction 
between plants and animals is, that plants prepare 
the food of animals by decomposing certain inorganic 
substances which animals cannot use as food, and 
recombining their elements into organic compounds 
