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 

 tlic Percfi. ^^ 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 



