FISHES. 
289 
hereditary that normal examples are rarely or never 
obtained ; such are the Jmvip-backed trout of a very isolated 
tarn of considerable depth, situated amongst the great hills 
of the Goberneasgach Deer Forest in Sutherland, which in 
all other respects are particularly vigorous and strong, and 
grow to a large size. The same variety — or deformity — we 
have often met with in less isolated areas. There " selec- 
tion " chose the more normal shape ; in the other case 
" isolation " produced the abnormal one. 
Peculiarities, scarcely to be called deformities, are in a 
similar manner reproduced by " isolation " — such as " b " 
and " d " of our List (;S^. cornuhiensis, Walb., and *S'. stomacM- 
cus, Giinth.), or the wonderfully beautiful small trout of the 
upper waters of the Smoo Burn, near Durness, in which 
the scarlet and sealing-wax-coloured blotches are almost 
confluent along the medial Ime, and are largely and boldly 
shown. Yet it is not so many years ago that this stretch 
of river was devoid entirely of any species of sabno, or any 
variety of fario. This wonderful variety was produced by 
" isolation " in so short a period of time as eight years. 
Where selection has nothing to select from, isolation 
steps in, and a permanent variety is in many cases 
rapidly constituted.^ 
Other varieties appear to gain their ascendency, not by 
" isolation," but by extended range and change of habitat, 
such as the peculiar tidal trout, or so-called " fossak " of 
the Inver and other rivers, named S. estuarius by Knox — 
variety " c " of our List. 
And yet another variety appears to gain specific dis- 
tinction — I mean the so-called S. ferox or great lake trout 
— simply by the development of carnivorous inclinations 
growing great teeth and a big wide-mouthed head ; losing 
1 We cannot here refrain from referring our readers to a little pamphlet by- 
Mr. Charles Dixon bearing upon these interesting natural phenomena, entitled 
Evolution ^vithout Natural Selection (R. H. Porter: London, 1885), views we have 
long cherished, but dared not to exjiress. 
