82 MARVELS OF FISH LIFE 
Changes in colour and markings are generally attri- 
buted to environment. A trout, for example, living in 
a deep hole of a brook, with a muddy bottom, is found 
to lose his bright spots, and to become of a dull colour, 
whereas the same fish living in a rapidly flowing stream 
with a gravelly bottom becomes bright in colour, and 
covered with numerous well-marked spots. 
Though environment undoubtedly has a material 
effect upon the coloration of the trout, food is an equally 
important factor in bringing about colour changes, and 
both colour changes and silvery iridescence may result 
from food and food alone. 
As an illustration of this point, I would quote from 
the experience of my friend, Mr. Richmond. 
On his fish farm at Shottermill are two similar ponds 
about one hundred and twenty yards apart. These 
were supplied from the same water source, but the food 
in them was of a totally different character. The upper 
pond abounded in molluscz, but the food in the lower 
pond consisted almost entirely of daphnia and cyclops. 
Both these ponds were stocked at the same time with 
trout similar in appearance. Very soon, however, the 
fish in the upper pond became dark in colour with in- 
tense red spots and yellow underparts, while those in 
the lower pond assumed a silvery appearance shot with 
a pale pink hue. 
The silvery appearance of the trout in the second 
pond, was due to deposits of light-reflecting spicules, 
known as iridocytes on the scales of the fish. When 
