200 JURA. LUMINOUS ANIMALS. 



warning to the male sex, though in the Lampyris, Ful- 

 gora, and Elater, both sexes give light: in the marine 

 animals it appears conducive to ends more universal, if 

 not more important; namely, to the general communi- 

 cation of all the inhabitants of the sea, for the immediate 

 object of self-preservation. 



From the experiments of Bouguer it is deduced that 

 the transmission of light through sea water is diminished 

 in a ratio so rapid, that at the depth of 723 feet it ceases 

 to be transmitted any longer. It is doubtful whether 

 these experiments are capable of determining so delicate 

 a question with such accuracy, yet the conclusions here 

 to be drawn from them will be the same, although that 

 depth should be far greater than the limit thus obtained. 

 At a certain depth therefore, which, if required, may 

 be admitted to exceed this three times, there is absolute 

 darkness. But this is by no means the limit in depth 

 of the inhabited ocean. Innumerable fishes are known 

 to exist in depths far greater than this, and to pass at 

 least a portion of their time in regions to which light 

 never penetrates.* Here they wander and prey; and 

 here apparently many tribes retire in winter, for reasons 

 which have not been discovered; although the fact is 

 well known to fishermen, who attribute it to the superior 

 warmth of the deeper water. In our own seas of little 

 depth, many fish are known to feed only by night ; appear- 

 ing, like beasts of prey, to sleep during the day. It is 

 not to be supposed that the regions of absolute darkness 

 in the ocean are uninhabited, since we are certain that 

 they are frequented by the pelagic fishes, and probably 

 by numerous species yet unknown to us. Nor is it pos- 

 sible that those which prey, either in these depths, or 



* In his recent voyage to the Arctic sea, Captain Ross brought up 

 shrimps by the sounding apparatus from a depth of 420 fathoms. Many 

 other animals of inferior organization were found at far greater depths. 

 These are unquestionably the prey of others, and the former, it is pro- 

 bahle, that of the whale. 



