6 



THE AQUARIUM. 



of a pond. Here we have a body of still water stocked 

 with a number of fish, snails, beetles, and a large variety 

 of other insects, all disporting themselves among a 

 mimic forest of water-plants. Here there is no change 

 in the water beyond what is caused by evaporation on 

 the one hand and by rain on the other. Yet in a pond 

 undisturbed by man the water is perfectly clear, and the 

 denizens enjoy good health and prosper. Having such 

 examples as the ponds before them, it is a wonder that 

 naturalists did not earlier set up aquaria on scientific 

 principles. But it was not until the year 1840 that these 

 principles were practically seized hold of and adopted. 

 About that date Dr. N. B. Ward published an account of 

 his experiments with plants in close glass cases — the 

 Wardian case now so extensively used for fern-culture. 

 He also varied the arrangement by filling the case with 

 water and placing therein living animals and plants. 

 He had discovered the law of mutual support in creation, 

 by which animals are dependent on plants and plants on 

 animals. He was followed a few years after by Mr. 

 Warington, Mr. P. H. Gosse, Drs. Bowerbank and Cotton, 

 and Dr. E. Lancaster, whose successes induced the 

 Zoological Society to erect an aquarium house in their 

 gardens in Regent's Park. Of what, it may be asked, 

 does this principle consist ? A very simple fact in natural 

 history, well known now to every school-boy, and yet 

 ignored so thoroughly by those who kept aquaria, when it 

 was fashionable so to do, and by many who aspire to 

 an aquarium as a mere ornament. Animals in the process 

 of respiration abstract oxygen gas from the air or water, 

 and give off carbonic acid gas. On the other hand, 

 plants absorb this carbonic acid gas, and by means of 

 their chlorophyll are able to fix the carbon contained in 



