44 BRITISH FRESH-WATER FISHES 
mammals and birds—an arrangement which has been shown 
by Professor Thayer to be protective, by reason that it 
neutralises the optical effect of that shade in which the lower 
parts of animals normally remain. It does more: it not only 
neutralises the shade, but, by means of the light reflected from 
the white underparts, it weakens the shadow cast upon the 
ground or bottom of the water by the body of the animal. 
Everybody who is practised in detecting salmon or trout in 
a river knows that the presence of the fish is more often 
betrayed by its shadow cast on the bottom than by a view 
of the fish itself. 
The Teleostean type of fish has developed so much vigour, 
and is endowed with such effective equipment for attack and 
Affinities of defence, as to secure for it a marked advantage in 
the Perch. the struggle for life; and among them, none have 
been more successful than the Perch Family, At the present 
time their genera and species, bewildering in number and 
variety, abound in all waters, both fresh and salt, throughout 
the temperate and tropical regions. 
It is interesting to trace the dominant features of the 
original type in the enormous variety of forms which have been 
evolved under different climatic and other physical conditions. 
The tendency to brilliant colouring, more or less suppressed in 
species inhabiting confined waters, breaks out in some of the 
marine perches into extravagant and startling combinations ; 
as if the armature of spines with which all members of the 
family are furnished enabled them to dispense with the protec- 
tive coloration accorded to less warlike creatures. 
Two members of the genus, Perca gracilis from Canada 
and Perca Schrenkii from Turkestan, differ very little from 
the species with which we are so familiar at home. The 
waters of Europe contain several kinds of perch which do 
not extend to Britain. In the apron (Aspro vulgaris) of 
the Upper Rhone and other rivers of Central France, the 
body has become strangely elongated and the head compressed 
