260 BRITISH FRESH-WATER FISHES 
too much upon these in a creature of such a plastic nature. 
The number of vertebrz, 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 cecal appendages have been cited 
repeatedly as a guide to species. Parnell, Yarrell, and Couch 
distinguished the trout of Loch Leven as Salmo cecifer, 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 caca 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 Sa/mo fario, lemanus, Rappit, and lacustris, 
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 Salmonida, p. 189. 
