DISTRIBUTION IN SPACE. 149 



devoid of the oxygen necessary for the support of animal 

 life. The sea-water is mainly oxygenated by the agitation 

 of the waves, and this extends but to a very limited depth 

 below the surface. It is now known, however, that the 

 depths of the ocean, though tranquil and undisturbed by 

 storms, are nevertheless renovated by a vast and com- 

 plex system of oceanic currents. In this way the oxy- 

 genated life -supporting surface-water is being constantly 

 transferred from the face of the deep to take the place of 

 the airless strata of the abysses, from which the oxygen 

 has been removed by the agency of living beings. 



Lastly, we have to consider how the animals of the deep 

 sea manage to exist in the absence of light. For plant-life 

 light is absolutely essential ; and though it is possible for 

 animals to exist in total darkness, the cases in which this 

 occurs are few, and the absence of light is generally ac- 

 companied by the loss of organs of vision. As a general 

 rule, however, light is all -important to animal life in its 

 higher developments, if only for the reason that without 

 light the predaceous animals could not see to capture their 

 prey. Without dogmatising as to the depth below the 

 surface to which light may penetrate, it seems certain that 

 Egyptian darkness must prevail at all depths below a few 

 hundred fathoms. This would at once account for the 

 absence in the deep sea of all vegetable life, with the 

 exception of such microscopic plants as most probably live 

 at or near the surface, and only fall to the bottom when 

 dead. Nevertheless, in the face of this, we find animals 

 living at a depth of more than two thousand fathoms with 

 perfect and well-developed eyes, as perfect as the organs of 

 vision possessed by animals living in illuminated regions. 

 It has been suggested by Sir Charles Lyell, as an explanation 

 of this fact, that the deep-sea animals are enabled to see by 

 their own phosphorescence. It is certain that many of them 

 phosphoresce brilliantly, and in the absence of any other 

 source of light it seems almost certain that they must owe 



