212 NATURAL HISTORY. 



depths greater than this animal life would be found to be 

 altogether wanting. 



Forbes's conclusions on this point met with the general 

 assent of naturalists, and it became an accepted doctrine 

 in zoology that animal life was wanting in the deeper 

 portions of the ocean. It would be out of place here 

 to detail the various steps which have led to an entire 

 reversal of Forbes's dictum on this point. Isolated 

 observations, both before and after Forbes's time, had 

 been recorded, which indicated the existence of animals 

 at great depths in the sea ; but these had been neglected 

 by naturalists, or had been looked upon with suspicion. 

 At present, however, the interest in these detached and 

 solitary observations is chiefly of an historical kind. Of 

 late years various nations, and notably our own, have 

 caused extensive and systematic explorations to be made 

 as to the physical and biological conditions prevailing 

 in the larger oceans. By various specially-equipped 

 expeditions the most important of which was that of 

 the Challenger the bottom of the deep sea has been 

 systematically explored by the dredge and trawl up to 

 depths of between three and four thousand fathoms, upon 

 a scale and with a completeness entirely beyond the reach 

 of any private individual. We now know that there is no 

 * zero of animal life' in the sea. So far from animals ceasing 

 to exist at depths greater than three hundred fathoms, they 

 are found at all depths in the sea. Nor are the animals of 

 the ' abyssal zone ' less numerous than those of shallower 

 waters. On the contrary, animal life exists in abundance 

 even at the greatest depths, where the temperature is 

 suitable; and it is the temperature of the water at the 



