characters, all related to habits or external conditions. We may say that
the third kind are correlated with some other character that has a
relation to function or external conditions, as the extension of the fins
on the under side of _Zeugopterus_ is correlated with the enlargement of
the fins, whose function is to cause the adhesion of the fish to a
vertical surface.

With regard to the specific characters of the species of _Zeugopterus_
nothing is known of peculiarities in mode of life which would give an
importance in the struggle for existence to the concrescence of the pelvic
fins with the ventral in _punctatus_, to the absence of this character and
the elongation of the first dorsal ray in _unimaculatus_, or to the
absence of both characters in _norvegicus_. No use is known for any of the
other specific characters, which tend in each case to form a series. Thus
in size _norvegicus_ is the smallest, _unimaculatus_ larger, and
_punctatus_ largest, the last reaching a of 8-1/2 inches. The subcaudal
fin-flaps are developed in _norvegicus_, most in _punctatus_; each has
four rays in _norvegicus_ and _unimaculatus_, six in _punctatus_. The
shortening and spinulation of the scales are greatest in _punctatus_,
least in _norvegicus_. In _punctatus_ there are teeth on the vomer,
in _unimaculatus_ none, in _norvegicus_ they are very small.

If we consider fishes in general, we see that there is no evidence of any
relation between many of the most important taxonomic characters and
function or external conditions. In the seas Elasmobranchs and Teleosteans
exist in swarming numbers side by side, but it is impossible to say that
one type is more adapted to marine life than the other. There is good
reason to believe that bony fishes were evolved from Elasmobranchs in
fresh water which was shallow and foul, so that lungs were evolved for
breathing air, and that marine bony fishes are descended from fishes with