10 THE HISTORY OF AQUARIA, 



CHAPTER II. 



THE HISTORY OF AQUARIA COntimied. 



THE most natural of all the artificial conditions under 

 which fish were kept in readiness for the table were 

 the old fish-ponds. Many of the latter were covered 

 with the usual aquatic vegetation, which thus kept the 

 water pure ; or else a stream regularly passed through 

 the lattice-work at either end. The relation which 

 plants and animals bear to each other was not fully 

 known even half a centuiy ago. It required con- 

 siderable progress in chemistry before the gases 

 which they gave off were understood. Unquestionably 

 the first step in this direction was made by Dr. 

 Priestley, of Birmingham, who observed that oxygen 

 gas was given off by plants when under the active 

 stimulancy of sunlight. Aquatic animals had been 

 described by Trembley, Baker, Leuwenhoek, Hooke, 

 and others ; but they either obtained them direct, or 

 else, as Trembley did his Hydras, kept them in jars 

 by constantly changing the water. Naturalists were 

 not aware of the needlessness of their labour until a 

 long period afterwards. Even Sir John Dalyell, whose 

 minute investigations into the structures and habits of 

 zoophytes were published in his splendidly illustrated 



