6 THE AQUAV1VAKIUM. 



ficial marine Aqitavivarium. In this arrangement 

 for cultivating plants and animals, fresh water is 

 converted into sea-water by simply adding the 

 common salt and other substances. Marine plants 

 and animals introduced into this liquid live as well 

 as in their native element. It is in this way that 

 we are able to cultivate in our drawing-rooms, not 

 only the living animals of rivers, and lakes, and 

 ponds, but those of the great ocean itself. 



Between sea-plants and animals and fresh- water 

 plants and animals there are many gradations ; 

 some forms requiring brackish water, some re- 

 quiring this mineral ingredient and others that ; 

 and a little study of the composition of water in 

 which plants and animals are found will enable 

 persons to succeed in the culture of even greater 

 varieties than have hitherto been attempted. 



In the above facts we have an illustration of the 

 principles which are necessary to secure life, but 

 nevertheless we cannot prevent death. It is one 

 of the characters of organic life, that its forms 

 should perish, and ample provision is made in the 

 structure of the plant and animal for the main- 

 tenance of the species. Both animals and plants 

 die, and the elements of which they were composed 

 are ultimately reduced to a state in which they 

 may again become the food of plants. But before 

 this takes place, a process of putrefaction sets in, 

 which has a power of spreading from the dead to 

 the dying and from the dying to the healthy, so 

 that putrefaction is a process to be avoided as 

 much as possible. In order to prevent this in the 

 great field of the world, certain animals are formed 

 who prefer dead to living prey, whose digestive 

 powers enable them to convert putrefying tissue 

 into the substance of their own bodies. Such 

 animals are the vultures and carrion crows 



