FIRST PRINCIPLES. 7 



amongst birds ; the crocodiles amongst reptiles ; 

 the sturgeons amongst fish ; the beetles amongst 

 insects ; and the water-snails amongst the Mollusca : 

 these are the scavengers of nature, and if man 

 imitated nature more closely, we should find a 

 larger number of scavengers in all our great towns 

 than we do at present. 



If we would, then, avoid mortality from putre- 

 fying substances which spread cholera and fevers 

 amongst our water pets, we must employ some sca- 

 vengers. These are best selected from the various 

 forms of Mollusca, It is, however, necessary in our 

 selection to remember that many of our molluscous 

 scavengers, like human ones, have a taste for some- 

 thing better than garbage, and unless due discrimi- 

 nation is used, our living plants as well as our dead 

 ones will fall a prey. 



Another point to be attended to in the manage- 

 ment of an Aquavivarium is the regulation of 

 temperature. With regard to the endurance of 

 change of temperature, man is altogether the most 

 remarkable animal he endures and flourishes 

 wherever other animals are found ; but the great 

 mass of the lower animals are made for special 

 temperatures. Those which dwell in polar regions 

 die on going north and south. The limits of the 

 extension of many animals are found within the 

 tropics. Species in the North Sea cannot, for the 

 heat, pass to the south, and vice versd. In like 

 manner, the. denizens of our water vivaries are 

 limited to certain temperatures, above or below 

 which they will not exist. Some will bear lower 

 and some higher temperatures better than others ; a 

 frost that will nip off all the Actince in a sea-tank 

 will produce no effect upon gold fish in a fresh- 

 water tank. The same with plants ; a frost that 

 will fatally blight Valisneria spiralis, will leave 



