vI COLORATION 171 
probably by a rat or a cat, the dark bands were permanently 
retained. 
Changes of coloration sometimes take place, which either have 
no discernible relation to age, condition, or surroundings, or are 
brought about by domestication; and in individuals of the same 
species there is often a wide range of colour-variation, which is 
sometimes, but not always, associated with particular localities. 
In some fresh-water Fishes a yellow colour may replace the 
original tint (xanthochroism). The usually dull greenish Tench 
(Tinea vulgaris) occasionally becomes a bright orange-yellow. 
Another Cyprinoid, the common Gold Fish (Cyprinus auratus), 
its wild state im China is also a dull brown or green, but, when 
domesticated, assumes in the first year of its life a black colour 
(melanism), then a silvery hue, and finally the vivid ruddy golden 
colour of the adult; occasionally, but rarely, the Fish is an 
albino. 
The value of a particular coloration in Fishes, either as an aid 
to concealment and protection from enemies, or by enabling them 
to secure their prey, may now be illustrated by a few examples. 
As previously shown, the colours of Fishes may be artificially 
varied according to their surroundings. Changes of a similar 
kind oceur naturally, and when they tend to assimilate the tints 
of the Fish to the prevalent hues of its surroundings, and con- 
sequently aid concealment, we have examples of what has been 
termed variable protective resemblance. Individuals of the same 
species vary in colour according to the opacity of the water they 
live in, becoming darker in muddy or peaty water, and brighter and 
lighter in shallower or clearer water. Trout caught in a stream 
with a gravelly or sandy bottom are lighter in colour than those 
obtained from a muddy stream, and it is well known that the 
same Fish changes colour as it passes from the one background to 
the other." Ina lake in County Monaghan, Ireland, the Trout are 
darker on that side which is bounded by a bog, but are of the 
beautiful and sprightly variety generally inhabiting rapid and 
sandy streams on the opposite side where the bottom is gravelly ; 
and narrow as the lake is, the two kinds of Trout appear to con- 
fine themselves to their respective areas.” Trout obtained from a 
1 Poulton, The Colours of Animals, Internat. Scientific Series, London, 1890, 
p. 82. 
2 Percy St. John, quoted by Day, Fishes of Great Britain and Ireland, London, 
1880-84, ii. p. 58. 
