86 The Colors of Fishes 
hues of these fresh-water fishes are, however, more or less con-. 
cealed in the water by the olivaceous markings and dark blotches 
of the upper parts. 
Coral-reef Fishes.—The brilliantly colored fishes of the trop- 
ical reefs seem, as already stated, to have no need of pro- 
tective coloration. They save themselves from their enemies 
in most cases by excessive alertness and activity (Chetodon, 
Pomacentrus), or else by burying themselves in coral sand (/Julis 
gaimard), a habit more frequent than has been suspected. 
Every large mass of branching coral is full of lurking fishes, 
some of them often most brilliantly colored. 
Fading of Pigments in Spirits—In the preservation of speci- 
mens most red and blue pigments fade to whitish, and it requires 
considerable care to interpret the traces which may be left of 
red bands or blue markings. Yet some blue pigments are abso- 
lutely permanent, and occasionally blood-red pigments persist 
through all conditions. Black pigment seldom changes in 
spirits, and olivaceous markings simply fade a little without 
material alteration. It is an important part of the work of the 
systematic ichthyologist to learn to interpret the traces of the 
faded pigment left on specimens he may have occasion to ex- 
amine. In such cases it is more important to trace the mark- 
ings than to restore the ground color, as the ground color is 
at once more variable with individuals and more constant in 
large groups. 
Variation in Pattern.—Occasionally, however, a species is 
found in which, other characters being constant, both ground 
color and markings are subject to a remarkable range of varia- 
tion. In such cases the actual unity of the species is open to 
serious question. The most remarkable case of such variation 
known is found in a West Indian fish, the vaca, which bears 
the incongruous name of Hypoplectrus unicolor. In the typical 
vaca the body is orange with black marks and blue lines, the 
fins checkered with orange and blue. In a second form the 
body is violet, barred with black, the head with blue spots and 
bands. In another form the blue on the head is wanting. In 
still another the body is yellow and black, with blue on the 
head only. In others the fins are plain orange, without checks, 
and the body yellow, with or without blue stripes and spots, and 
