1894-95-] Do Trout Purify or Pollute Water? 127 



have of imparting information which may be new to the 

 members. 



Having a small aquarium in my house in which I have 

 from time to time kept various species of fishes, as also frogs, 

 newts, molluscs, gammari, &c, I have — though of course in 

 artificial circumstances — had opportunities afforded me of 

 studying the habits and peculiarities of these different 

 creatures. 



Before the law of gravitation was understood, the inhabi- 

 tants in many cases depended on a well for their water-supply, 

 and even at the present day in many country villages the 

 same thing exists. A popular notion prevails that a trout in 

 the well is indispensable in order that it may eat up any 

 animalculae that might constitute pollution, and my observa- 

 tions and experiments throughout life very generally corrobo- 

 rate this theory. Those who have been at the trouble to 

 observe a filter-bed after the water has been run off must 

 have been struck with the correctness of this opinion. I have 

 frequently been interested in watching the water being run off 

 from one of the filter-beds of the water-supply of our own 

 city. Despite the number of trout, eels, pike, and perch in 

 the reservoir, the quantities of Limnseas, Gammari, &c, are in- 

 credible. To those who know the immensely prolific nature 

 of these aquatic creatures, and the amount of larva? and spawn 

 which they produce, the gross pollution they would cause with- 

 out the aid of scavengers in the shape of fish must be very 

 apparent. As an experiment, I put half-a-dozen molluscs 

 {Limncca peregra) into a crystal tumbler among water clear as 

 the crystal itself, and in twelve hours the water was very 

 sensibly muddy. Some capsules containing the eggs of the 

 molluscs adhered to the sides of the tumbler. When it is 

 taken into consideration that this happened without the 

 Limnaeae having anything to eat, it will be readily believed 

 that in natural circumstances, with abundance of food, the 

 pollution must have been far greater. 



That Limnseae, Gammari, &c, constitute the food of small 

 trout is well known. It is a never-failing law of nature that 

 wherever any creatures are found which multiply fast, checks 

 are also found by way of counterbalance. Divest a lake of 

 fish and the creatures referred to would increase amazingly, 



