MAY 101 



or taking the trouble of trying to do so. For instance, 

 why is it of primary importance for a fish to keep on 

 an even keel, and how does it manage to do so, even 

 in swift and rough water ? 



' When one realises how very slightly a fish has to turn on 

 its side to reveal itself, it is remarkable that we so seldom 

 see a silvery glint as he dashes through the water. Watch 

 trout shooting off a shallow into the pool above. It is 

 possible you may detect the shadowy form ; but more often 

 than not the only indication is the point of the wedge-like 

 ripple as he leaves the gravelly bed. When fish swim 

 rapidly they do so by swishing their tails from side to side, 

 and there certainly would be sufficient roll of the body for 

 the silvery sides to catch the light from above, were it not 

 for the fact that a fish when he swims fully extends his 

 dorsal and anal fins, and these, acting like the keel of a boat, 

 keep him in a straight course [1 keep his sides perpen- 

 dicular].' 1 



Reference has been made above to the automatic 

 change of colour in certain fishes to assimilate it to 

 their surroundings. This phenomenon was brought 

 particularly to my notice in the following manner: 

 Having received from Germany (many years before the 

 war, be it said !) a quantity of yearling black bass with 

 which I hoped to stock a lake in the park, they were 

 placed on arrival in a pond, there to remain till such 

 time as they might be deemed of a size to take their 

 lives in their hands, so to speak, in water infested with 

 pike. We gave them two years' grace and then started 

 to net them in the pond. Unluckily, the bottom was 

 cumbered with stumps and snags, and the net would 

 not work properly. In the first draught we only got 



' Ibid., p. 25. 



