252
THE BIOLOGY OF FISHES
'
intense, ‘asin the tr0pics, the colours are vivid ; where both
'
are
in the dark caves of Cuba and the States,
.
cOlOurs may be quite wanting and even the pigment of the
retina does not develop.
Cunningham has proved also
j
'
that the under-surface of Flat—shes, normally white,
7
becomes coloured when the rays of light are brought to
bear upon it.
Inshore shes living among variegated
surroundings are more coloured than offshore shes, and
the deep-sea forms are either silvery or black. If the
'_
colour-is any protection it comes from the fact that the
'
pigment absorbs the dangerous rays oflight (Buchmann).
-
.
How difcult it is to regard colour as an adaptation, other
than in the above sense, Can be seen from a comparison of the
different colours of shes living under the same conditions.
,
It has been
that the slaty blue of the back and
,
‘
white'of'the' belly in many shes, like the Cod, render them
more; or less invisible eto enemies
above or below.
_
'
Yet the Mackerel and Herring are conspicuous in the same
.
-
=Waters__ and other shes may be almost black (Cantharus)
Ï
Ÿ
.
or even red (Gurnards and Red Mullet).
—
the other hand, if we. meet with cases. where the
sh appears
to .,make' a denite Use of its resemblanëe to
*
particular"surroùndirigs, we can say that it adapts itself
thereto. In the summer months the PlaiCe :chooses the
bright, clean sand -for its residence, whilstthe darkerDab
_
aIid Flounder prefer mud. : But in the spring—time;whenit _ ,
;
has _spawned and is o Colour Or 'Out}_bf “condition,—: the
,
hauts the ‘mùddy£_groùrid. }Theidifferent spéCies”of
_
,
.
5
between
;
;
”origin Of a chàracter,’i£ 1pœÇ$i-ble, and
to which it is
:
different uses;
,
'
:, _
-,
tail Oracaudalz'nfdeVelopssas
providing '
an equipoise
to the head; r,Where no air—bladder“ is present
'
andjwhere
the c‘entreof gravity lies.near;th<ä,head;thé tail
'upwards (hetemCercy)“and thus: by… its movements.
Î
foraise the
’
That—is the gen—era—l'purpoSeg
_