336 TRANSACTIONS, NATURAL HISTORY SOCIETY OF GLASGOW. 
general lines I would place the fact that deep-sea fishes and 
bottom fishes are less highly coloured than those of shallow 
seas and surface swimmers. If we take our cod family as 
bottom swimmers, and the mackerel as surface, to an extent, 
we have a fairly good common set of examples. And again, on 
the same grounds, I should say that off-shore fishes are less highly 
coloured than those nearer the tangle zone, where light is more 
plentiful and accessible. 
There are numerous side issues to this general principle, and 
although at first sight such principle may alone be responsible, yet 
it may not! Thus, although the upper portions of fishes are 
richer toned, and the bellies deficient in colour, if not entirely 
devoid of it in a general sense, the direct action of light may not 
really be the primary, and, still less, the sole cause. At the same 
time, when we take the flat-fish as an example, they would, as a 
group, prove that so soon as they turn one side away from the 
light that side loses its pigment and soon becomes non-coloured, 
however brilliantly the upper and exposed surface may be tinted 
or hued. The many individual instances to the contrary, in which 
our commoner flat-fishes are semi-coloured or, occasionally, even as 
richly parti-coloured on the lower as on the upper surface—and 
these are commoner than are generally supposed—are probably 
merely reversions to an original type, when both sides were equally 
illuminated. They can scarcely be looked upon as having any- 
thing to do with light in the special specimen captured. Even 
this, however, is not an impossibility, as was shown to an extent 
by Mr. J. T. Cunningham, in his interesting experiments in 
rearing flat-fish with lights thrown upon them from below as well 
as from above. I am more disposed to look upon the ordinary 
exceptions produced as merely reversions, Yet in the so far 
analogous instances of some of the Echinodermata, such as Pal- 
mipes, which has often a brilliant band on the lower surface, 
usually hidden in the mud, they have a habit of occasionally 
curling up, which gives them a possible opportunity of colouring 
the outer fringe. In this position they much resemble a transition 
stage to the sea-urchin—which, indeed, they are supposed to be. 
After the general principle of direct sunlight comes the secondary 
principle of surroundings and ground frequented. This may 
occasionally be attributable, either directly or indirectly, to the 
