26o BRITISH FRESH-WATER FISHES 



too much upon these in a creature of such a plastic nature. 

 The number of vertebras, for instance, is often relied upon as 

 an indication of species, yet the uncertainty of this test is manifest 

 if we accept Dr. Day's assurance that he has found these vary 

 in British fresh-water trout from fifty-six to sixty.* Again, 

 the number and size of cascal appendages have been cited 

 repeatedly as a guide to species. Parnell, Yarrell, and Couch 

 distinguished the trout of Loch Leven as Salmo cacifer^ because of 

 the superior development of these appendages to the intestine ; 

 but here again there is nothing like fixity. Common Scottish 

 trout have been found with as few as twenty-seven, and as 

 many as sixty-nine ; while in Loch Leven trout themselves they 

 have been found to vary between forty-eight and ninety. Now 

 these pyloric caeca are secreting tubes, closed at the outer end, 

 situated along the small intestine, and their function is supposed 

 to be that of a supplementary pancreas. It is probable that 

 their development varies, not according to the species, but in 

 proportion to the amount and quality of food. Where food 

 is deficient, there will be the less exercise for these organs, 

 which will show a tendency to diminish in size and number ; 

 where it is abundant, rich, and stimulating, they will tend to 

 develop as aids to nutrition. 



These considerations have led me to adopt the view of those 

 ichthyologists who regard all British fresh-water trout, if not 

 all European ones, as varieties of a single species. Lunel 

 proposed to include all the four so-called species of Lake 

 Constance, defined as Salmo fario^lemanus^ Rappiiy and lacustriSy 

 under the single title of Salmo variabilis^ and in truth no more 

 appropriate title could be desired ; because not only do these 

 fish acquire modified organic characteristics under the influence 

 of food, its quality and abundance, and of such physical 

 environment as soil, water, and climate, but fish reared from 

 eggs out of the same ovary undergo such rapid change of 

 colour and markings according to the bottom they swim over, 

 * British and Irish Salmonidce, p. 189. 



