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In the case of any of the lower animals an enquiry regarding its power
of appreciating different colours is usually conducted by making
experiments calculated to show whether or not the animal in question
behaves as if it had this power. Whether it actually sees colours as we
do, or is even conscious of seeing them at all, is, necessarily, beside
the question.
So far as this method of investigation carries us in the case of the
salmon, there would seem, from what is mentioned elsewhere, to be
trustworthy evidence that it is influenced by differences in the colour
of the artificial fly. But there remains a considerable difference of
opinion, I understand, as to the value of these practical observations.
That the different colours and light intensities of an object, situated
within a definite area of the water surface, are theoretically visible
by the fish’s eye, has been clearly shown by Mr. J. Allan Stewart, who
has thus answered those who argued that any body on the surface of the
water would only be seen as a dark object against the bright sky-ground.
The fact of an animal choosing one part of the spectrum in preference to
another can only be accepted as evidence that it recognises some
difference. Now, it is only from the facts known regarding human vision
that we can draw comparative conclusions. Judging from these facts we
find that certain parts of the spectrum affect us differently from other
parts, not only by virtue of their colour, but also of their brightness.
We find, indeed, that we have two distinct visual sensations dependent
on light, _colour_ and _brightness_, both influenced by differences in
wave-rapidities or wave-lengths. Thus, if we gradually reduce the
intensity of the spectrum, the colours all finally cease to be
recognisable. In this colourless spectrum the brightest part is slightly
to the violet side of the line E (corresponding to green), so that it is
quite possible that this part of the spectrum would appear distinct from
the rest, even in the case of an animal destitute of all colour
perception. (In proof of this it may be mentioned that in a case of
total congenital colour-blindness in man, this part of the spectrum is
found to be recognised as different from the rest by its brightness
alone.)
From arguments which I have adduced elsewhere, there is good evidence in
favour of the view that light perception (brightness) is the more
primitive of these two sensations, and that it is dependent upon changes
induced by light in the retinal pigment epithelium. Also, it is
practically proved, so far as the human retina is concerned, that this
light impression is communicated to the _rod-cells_ of our retinal
neuro-epithelium and so transmitted by conducting channels in retina,
nerve and ganglion, to the sight centre in the brain. In the case,
therefore, of an animal with a low form of retina, in which the
above-named cells are alone represented, any differences in its
behaviour on exposure to different parts of the spectrum may be presumed
to be explained by the amount of brightness or light effect thereby
induced.
But, on the other hand, we now believe that the finer gradations of
differences in stimulation due to variations in wave-length which we
recognise as _colours_ are dependent on the presence of another kind of
retinal element called _cone-cells_. It is sufficient here merely to say
that these cone-cells seem to derive the initial stimulus through some
effect produced on the pigment epithelium, and that they also transmit
the impulse by conducting paths to the brain in a similar fashion to
that above-mentioned.
Accordingly, should the retina of an animal contain, not only pigment
and rod-cells, but also cone-cells, we must admit that, so far as we
know, it is anatomically and physiologically capable of being influenced
by colour in a similar way to the human retina. Now, the retina of the
salmon does undoubtedly possess all these anatomical elements—pigment
epithelium, fine rod-cells, well-marked cones.
The whole argument for the existence of colour perception in the salmon
is, therefore, undoubtedly a strong one.
(1) It behaves as if it sees differences in colour.
(2) It is possible for its eye to get a proper view of an object within
a certain area on the surface of the water.
(3) Its retina is of a sufficiently highly developed type to admit of
its being differently influenced by different parts of the colour
spectrum.