238 Mr Saunders, A Note on the Food of Freshwater Fish. 
crustacea. Thus we see that the food of the stickleback may 
vary, but the variation is not a haphazard one for it effects equally 
all the individuals that live in one pond. It is something in the 
conditions under which a fish lives that determines its preference — 
for a certain kind of food. 
Under artificial conditions I found that the feeding habits of 
the fish completely changed. I could never get the large forms 
which fed on Diatoms to eat Diatoms, and I suspect this was 
because the Diatoms did not grow in their natural manner. These 
larger forms would however become carnivorous in aquaria and 
gave the impression that this was their natural method of feeding. — 
Experiments in the laboratory are therefore useless to determine 
this point. 
The fisherman, who catches fish with bread paste and gaudy 
flies, will probably ask how it is that such a thing as his bait 
which the fish cannot possibly have ever seen before proves so 
attractive. But the bait is not an article of diet. Every fisher- 
man knows that there are times when the fish are feeding and 
times when they are not. Now my experiment of introducing 
Copepods into an aquarium and thus getting sticklebacks to snap 
at waterbeetles, shows that when fish are “on the feed” they will 
snap at anything that catches their eye, but unless they seize 
something palatable they will not swallow it. Therefore the 
fisherman angles when the fish are “on the feed,” that is moving 
about in search of food, and he uses as a bait something that is . 
likely to catch the fish’s eye. The fish snaps at the bait and the 
fisherman strikes so as to drive the hook into its mouth. If the 
bait used were something that the fish ordinarily ate there would 
be no need for the exercise of any skill, for the fish would swallow 
it and inevitably get hooked. As it is the fisherman has to strike 
at the moment the fish snaps and before it has a chance to reject 
the bait. Herein lies the sport of fishing, for the more tentative 
the snap the greater will be the skill required to hook the fish ; the 
fish that snap so vigorously that they will hook themselves provide 
little sport for the angler. 
We therefore come to two conclusions. 
(i) Fish may feed on different things, but a meal always 
consists of one class of food only. This means that in order to 
ascertain the food of a fish we must open stomachs of fish taken 
at different seasons and under different conditions. 
Additional evidence on this point is afforded by an observation, 
Dakin and Latarche (Proc. of Royal Irish Ac., Vol. Xxx. See. B, 
No. 3). They examined the contents of the stomachs of large 
numbers of Pollan (Coregonus pollan) and they remark: “The 
most extraordinary thing however was that on most occasions 
when Daphnids were found in the alimentary canals, practically 
