THE PERCH 43 
they arise from emotional excitement, as is the case with the 
rock goby (Gobius niger), a marine fish, which turns from 
ashen grey to a dark, angry smoke-colour when in pursuit of 
prey. In the perch, as in the common trout and many other 
fish, the skin responds sympathetically and automatically, but 
by a mechanism not hitherto satisfactorily explained, to the 
various colour rays reflected from surrounding objects. 
The degree in which fish of the Perch Family possess this 
sensibility to colour environment, however temporary, happens 
to have impressed itself upon my attention more forcibly in the 
case of another member of the family, namely, the American 
black bass (Micropterus salmoides), than in that of the common 
perch. The black bass bears a strong general resemblance to 
the British perch. Some years ago, having reared a number 
of these bass from the yearling stage in a disused mill-pond, 
and desiring to turn them into a large lake for naturalisation, 
I had the said pond drawn with a net. The first draught brought 
out half a dozen bass nine or ten inches long. The general tint 
of these fish on back and sides was a rich dark olive-green. 
Placed in a wash-tub, whereof bottom and sides had been 
bleached to a whitish hue, they appeared very dark indeed. 
A cloth was laid over the tub to prevent the bass leaping out, 
and the net was made ready for a second draught. In fifteen 
or twenty minutes, when some more fish were brought to the 
tub, the first lot had completely changed their tint, being 
closely assimilated to their new background. 
While treating of the perch, I have dwelt in some detail 
upon this phenomenon of colour change, not because it is 
the only fish of that family, but because it is the 
only British fish in which the obvious intention of such 
change, namely, concealment, is defeated by the permanence 
of conspicuous markings. In other respects, similar aids to 
invisibility are present in the perch as have been observed in 
other vertebrates, both terrestrial and aquatic. For instance, 
the belly of this fish is pure white, as is the case with so many 
