structures at all, although of great importance as diagnostic characters.
It may be urged that in some cases, such as the little _Agonus
cataphractus_ or the Seahorse among the Syngnathidae, the body is
protected by a complete suit of bony armour; but accompanying these in the
littoral region are numerous other species such as the Gobies, and even
other species of Syngnathidae which have soft unprotected skins.

Similarly with colour characters: the power of changing the colour so as
to harmonize with the ground is obviously beneficial and adaptive, but in
each species there is a specific pattern or marking which remains constant
throughout life and has nothing to do with protective resemblance,
variable or permanent. The red spots of the Plaice are specific and
diagnostic, but they confer no advantage over the Dab or the Lemon-dab, in
which they are absent, nor can any relation be discovered between these
spots and mode of life or habits.

The function of the lateral line organs is still somewhat obscure. The
theory that they are sensitive to differences of hydrostatic pressure as
the fish moves from one depth to another rests on no foundation, since it
has yet to be shown how a change of pressure within the limits of the
incompressibility of water can produce a sensation in an organ permeated
throughout with water. It is more probable that the organs are affected by
vibrations in the water, but we are unable to understand how a difference
in the anterior curvature of the lateral line would make a difference in
the function in any way related to the difference in conditions of life
between Plaice and Dab. There is, however, reason to conclude that the
organs, especially on the head, are more important and larger in deeper
water, and thus the enlargement of the sensory canals in the head of the
Witch, which lives in deeper water than other species, may be an
adaptive character.