GENERAL PRINCIPLES. 



7 



it, and set free the oxygen. So that, leaving all other 

 points of dependence out of the question, animals could 

 not long exist in an atmosphere unpurified by plants ; 

 neither could plants maintain a healthy existence if 

 animals did not pollute the atmosphere with their car- 

 bonic exhalations. 



If, then, this be a natural law, universal in its opera- 

 tion, it is easy to understand why live fish and other 

 aquatic animals were kept with such difficulty when 

 aquatic plants were entirely excluded from the tanks. 

 It is still the fashion to keep tolerably large gold-fish in 

 comparatively small glass globes, without an atom of 

 water-weed, and consequently the water is soon robbed 

 of its oxygen, and, therefore, rendered incapable of sus- 

 taining animal life. Under these circumstances it is 

 necessary constantly to change the water, which also 

 implies a continual change in the temperature surrounding 

 the unlucky fish. The life of the poor creatures kept 

 under such circumstances is most unnatural. Surrounded 

 by a glare of light from all sides, with neither stones nor 

 weeds under whose shade to rest, and with scarce room 

 to turn round, they can but lie motionless at the bottom 

 of the vessel, or make a weary endless pilgrimage round 

 its circumference. 



The aquarium-keeper should, above all things, strive to 

 copy nature as far as possible. The light which pene- 

 trates the water should come from above, not from the 

 side. There should be one or two corners in semi- 

 darkness. There should be an abundance of fresh living 

 plants. The luater should never be changed. Care should 

 be taken that the animals and plants are pretty equally 

 balanced, and that all are in proportion to the bulk of 

 water. 



