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species belonging to each being unable to exist in the 

 medium proper for the other, excepting some who pass with 

 impunity from one into the other, at pleasure, or during cer- 

 tain seasons. In other respects, few natural families are 

 without some genus or species to represent the forms and 

 duties of its congeners in every sea. It is true, that we are 

 not acquainted with what species, or in what numbers, the 

 great depths of the ocean are more particularly inhabited ; 

 but we may infer by analogy, from the conditions of existence 

 in all the vertebrated animals, that life under a continually 

 increasing pressure, in proportion to the depth of the super- 

 incumbent column of water, must at a given point reach the 

 limit where eternal darkness renders the organs of sight un- 

 availing, and consequently the power of obtaining or avoid- 

 ing prey impossible ; still lower, where all the action of 

 animal life must cease, where the gravity of no animal matter 

 will descend, and finally, where even metals must remain 

 suspended. Many atmospheres of water above these, we 

 may therefore conclude to be the region where fish in a 

 natural state can reside ; comparatively, in short, at no great 

 depth, and possibly not far below one hundred fathoms. 

 For the bottom of the sea already, before reaching to such 

 depth and lower beneath it, no longer offers, or at least 

 scarcely offers, to the observer on the deep sea lead, aught 

 except broken shells, teeth of fish, sand, and rock. No nets 

 exceeding half that depth are anywhere in use, and the fish 

 which are sometimes caught at fifty fathoms below the sur- 

 face, are in general of species provided with eyes of such 

 magnitude as to indicate the probability that their enlarged 

 organs of sight are necessary in a medium so dense and 

 remote from the light. Light, the manifestation of the solar 

 action, is necessary in a greater or less degree, diurnally or 

 at. greater intervals, to the whole of organic nature ; the spe- 

 cies, therefore, which periodically rise from the deep, and 



