Pvn H,i I'UitlS

314 CYPRINID^.
g-round : doubtful specimens occur where they might be
looked for— viz. on the confines of two localities, each
inhabited by its own form. The above remarks equally
apply to the A. incrassata of Brocchi, except in its being
a southern form. Philippi at first considered it the same
as our species, but he afterwards changed his opinion
on the authority of M. Koch. Deshayes, in his edition
of Lamarck, represents A. incrassata and the Tellina
fasca of Poli as two distinct species ; but Philippi demurs
to this, on the ground that the extent of sulcation and
the comparative convexity of the shell are very uncer-
tain characters. Such a conflict of opinion is easily
settled by pronouncing the three species to be distinct ;
but I believe the sounder judgment will be that all are
identical. It is impossible to ascertain, or even to
conjecture with any reasonable probability,
how much
change prolific and hardy species, like the present, may
have undergone during the lapse of countless genera-
tions ;
" For formes are variable, and decay
By course of kind and by occasion."
The A. elliptica of the north, A. sulcata of more tem-
perate seas, and A. incrassata of the south are evidently
so closely allied to one another that it is not unphilo-
sophical to suppose that they originated from the same
common stock. The slight differences which they pre-
sent among themselves may have been caused by local
or accidental conditions. I need not apologize for par-
ticularizing so many varieties,
as all naturalists are
of this mode of discrimination .
agreed as to the utility
The time has gone by when varieties were not regarded.
At present the course of scientific inquiry tends the
other way ;
and varieties must be named, or have some
equivalent symbol of distinction.