DISTRIBUTION IN SPACE. I47 



fore, we are to retain the four zones above mentioned, we 

 must now add to these a fifth or Abyssal zone, extending 

 from 100 fathoms up to at least 2500 fathoms, and doubt- 

 less really extending to all depths in the ocean. 



The most important result, however, of these inquiries is 

 the discovery of the fact that, beyond a very limited depth, 

 the distribution of marine animals is conditioned, not by the 

 depth of the water, but by its temperature. Thus the bat/iy- 

 metrical distribution is truly a thermometrical one. Similar 

 forms, namely, are found inhabiting areas in which the 

 bottom-temperature is the same, wholly irrespective of the 

 depth of water. It may happen, therefore, that two dis- 

 tinct faunae may inhabit contiguous areas of the sea-bottom, 

 and may be even sharply marked off from one another, as 

 when on.e area is swept by a warm current, whilst a neigh- 

 bouring area has its temperature lowered by a cold current. 



The conditions under which the animals of the deep sea 

 live, are so different to those to which the inhabitants of 

 shallow waters are subjected, that a few remarks upon this 

 subject may advantageously be added here. 



It was formerly beHeved that the pressure of the water at 

 great depths would be so enormous as to preclude the pos- 

 sibility of life being present. This, however, is a fallacy ; 

 since the internal pressure of any body immersed in a fluid, 

 and admitting fluid into its interior, is in all cases the exact 

 equivalent of the external pressure. In other words, marine 

 animals are in this respect- in the same position as an un- 

 corked bottle sunk at the bottom of the sea. AVhatever the 

 depth may be, there is no pressure upon the sides of the 

 bottle, because the pressure of the water outside the bottle 

 is exactly neutralised by the pressure of the water in its 

 interior. 



In the second place, it is a well-known generalisation 

 that animals are, mediately or immediately, dependent 

 upon plants for their subsistence. Plants, however, can- 

 not exist unless supplied with solar light, and there is 



