THE COLOUR SENSE.
The previous considerations lead us, naturally, to enquire in what
manner the sense of colour is perceived.
In thinking over this obscure subject, the opinion has steadily gathered
strength that form and colour are closely allied; for form is essential
to pattern; and colour without pattern, that is to say, colour
indefinitely marked, or distributed, is hardly decoration at all, in the
sense we are using the term. That many animals possess the power of
discriminating form is certain. Deformed or monstrous forms are driven
from the herds and packs of such social animals as cattle, deer, and
hogs, and maimed individuals are destroyed. Similar facts have been
noticed in the case of birds. This shows a power of recognising any
departure from the standard of form, just as the remorseless destruction
of abnormally coloured birds, such as white or piebald rooks and
blackbirds, by their fellows, is proof of the recognition and dislike of
a departure from normal colouring. Authentic anecdotes of dogs
recognising their masters' portraits are on record; and in West Suffolk,
of late years, a zinc, homely representation of a cat has been found
useful in protecting garden produce from the ravages of birds. In this
latter case the birds soon found out the innocent nature of the fraud,
for we have noticed, after a fortnight, the amusing sight of sparrows
cleaning their beaks on the whilom object of terror. Many fish are
deceived with artificial bait, as the pike, with silvered minnows; the
salmon, and trout, with artificial flies; the glitter of the spoon-bait
is often most attractive; and mackerel take greedily to bits of red
flannel. Bees sometimes mistake artificial for real flowers; and both
they and butterflies have been known to seek vainly for nourishment
from the gaudy painted flowers on cottage wall-papers. Sir John Lubbock
has demonstrated the existence of a colour sense in bees, wasps, and
ants; and the great fact that flowers are lures for insects proves
beyond the power of doubt that these creatures have a very strong
faculty for perceiving colour.
The pale yellows and white of night-flowering plants render them
conspicuous to the flower-haunting moths; and no one who has ever used
an entomologist's lantern, or watched a daddy-long-legs (_Tipula_)
dancing madly round a candle, can fail to see that intense excitement is
caused by the flame. In the dim shades of night the faint light of the
flowers tells the insects of the land of plenty, and the stimulus thus
excited is multiplied into a frenzy by the glow of a lamp, which,
doubtless, seems to insect eyes the promise of a feast that shall
transcend that of ordinary flowers, as a Lord Mayor's feast transcends a
homely crust of bread and cheese.
We take it, then, as proven that the colour sense does exist, at least,
in all creatures possessing eyes. But there are myriads of animals
revelling in bright tints; such as the jelly-fishes and anemones, and
even lower organisms, in which eyes are either entirely wanting or are
mere eye-specks, as will be explained in the sequel. How these behave
with regard to colour is a question that may, with propriety, be asked
of science, but to which, at present, we can give no very definite
reply. Still, certain modern researches open to us a prospect of being
able, eventually, to decide even this obscure problem.
The question, however, is not a simple one, but involves two distinct
principles; firstly, as to how colour affects the animal coloured, and,
secondly, how it affects other animals. In other words, How does colour
affect the sensibility of its possessor? and how does it affect the
sense organs of others?
To endeavour to answer the first question we must start with the lowest
forms of life, and their receptivity to the action of light; for, as
colour is only a differentiation of ordinary so-called white light, we
might _a priori_ expect that animals would show sensibility to light as
distinguished from darkness, before they had the power of discriminating
between different kinds of light.
This appears to be the case, for Engelmann has shown[9] that many of
the lowest forms of life, which are almost mere specks of protoplasm,
are influenced by light, some seeking and others shunning it. He found,
too, that in the case of _Euglena viridis_ it would seek the light only
if it "were allowed to fall upon the anterior part of the body. Here
there is a pigment spot; but careful experiment showed that this was not
the point most sensitive to light, a colourless and transparent area of
protoplasm lying in front of it being found to be so." Commenting upon
this Romanes observes, "it is doubtful whether this pigment spot is or
is not to be regarded as an exceedingly primitive organ of special
sense." Haeckel has also made observations upon those lowest forms of
life, which, being simply protoplasm without the slightest trace of
organization, not even possessing a nucleus, form his division
_Protista_, occupying the no-man's-land between the animal and vegetable
kingdoms. He finds that "already among the microscopic Protista there
are some that love light, and some that love darkness rather than light.
Many seem also to have smell and taste, for they select their food with
great care.... Here, also, we are met by the weighty fact that
sense-function is possible without sense organs, without nerves. In
place of these, sensitiveness is resident in that wondrous,
structureless, albuminous substance, which, under the name of
protoplasm, or organic formative material, is known as the general and
essential basis of all the phenomena of life."[10]
Now, whether Romanes be correct in doubting whether the pigment-spot in
Euglena is a sense organ or not, matters little to our present enquiry,
but it certainly does seem that the spot, _with its accompanying clear
space_, looks like such an organ. And when we are further told that
after careful experiment it is found that _Euglena viridis_ prefers blue
to all the colours of the spectrum, the fundamental fact seems to be
established that even as low down as this the different parts of the
spectrum affect differently the body of creatures very nearly at the
bottom of the animal scale. This implies a certain selection of colour,
and, equally, an abstention from other colours.
It is not part of our scheme, however, to follow out in detail the
development of the organs of special sense, and the reader must be
referred to the various works of Mr. Romanes, who has worked long and
successfully at this and kindred problems. Suffice it to say that in
this and other cases he has been led to adopt the theory of inherited
memory, though not, as we believe, in the fulness with which it must
ultimately be acquired.
This, however, seems certain, that the development, not only of the
sense organs, but of organs in general--that is, the setting aside of
certain portions for the performance of special duties, and the
modifications of those parts in relation to their special duties, is
closely related to the activity of the organism. Thus, we find in those
animals, like some of the Coelenterata, which pass some portion of their
existence as free-swimming beings, and the remainder in a stationary or
sessile condition, that the former state is the most highly organized.
This is shown to a very remarkable degree in the Sea Squirts
(Ascidians), a class of animals that are generally grouped with the
lower Mollusca, but which Prof. Ray Lankester puts at the base of the
Vertebrata.
These animals are either solitary or social, fixed or free; but even
when free, have little or no power of locomotion, simply floating in the
sea. Their embryos are, however, free-swimming, and some of the most
interesting beings in nature. Some are marvellously like young tadpoles,
and possess some of the distinctive peculiarities of the Vertebrata.
Thus, the body is divided into a head and body, or tail, as in tadpoles.
The head contains a large nerve centre, corresponding with the brain,
which is produced backwards into a chord, corresponding to the spinal
chord. In the head, sense organs are clearly distinguishable; there is a
well-marked eye, an equally clear ear, and a less clearly marked
olfactory organ. Besides this, the spinal-cord is supported below by a
rod-like structure, called the notochord. In the vertebrate embryo this
structure always precedes the development of the true vertebral column,
and in the lowest forms is persistent through life.
We have thus, in the ascidian larva, a form which, if permanent, would
most certainly entitle it to a place in the vertebrate sub-kingdom. It
is now an active free-swimming creature, but as maturity approaches it
becomes fixed, or floating, and all this pre-figurement of a high
destiny is annulled. The tail, with its nervous cord and notochord
atrophies, and in the fixed forms, not only do the sense organs pass
away, but the entire nervous system is reduced to a single ganglion, and
the creature becomes little more than an animated stomach. It is, as Ray
Lankester has pointed out, a case of degeneration. In the floating
forms, which still possess a certain power of locomotion, this process
is not carried to such extremes, and the eye is left.
Now, cases of this kind are important as illustrating the direct
connection between an active life and advancement; and they also add
indirectly to the view Wallace takes of colouration, namely, that the
most brilliant colour is generally applied to the most highly modified
parts, and is brightest in the seasons of greatest activity.
But they have a higher meaning also, for they may point us to the prime
cause of the divergence of the animal and vegetable kingdoms. In
thinking over this matter, one of us ventured to suggest that probably
the reason why animals dominate the world, and not plants, is, that
plants are, as a rule, stationary, and animals lead an active existence.
We can look back to the period prior to the divergence of living
protoplasm into the two kingdoms. Two courses only were open to it,
either to stay at home, and take what came in its way, or to travel, and
seek what was required. The stay-at-homes became plants, and the
gad-abouts animals. In a letter it was thus put; "It is a truly strange
fact that a free-swimming, sense-organ-bearing animal should degenerate
into a fixed feeding and breeding machine. It seems to me that the power
of locomotion is a _sine qua non_ for active development of type, as it
necessarily sharpens the wits by bringing fresh experiences and
unlooked-for adventures to the creature. I almost think, and this, I
believe may be a great fundamental fact, that the only reason why
animals rule the world instead of plants is that plants elected to stay
at home, and animals did not. They had equal chances. Both start as
active elements; the one camps down, and the other looks about him."
Talking over this question with Mr. Butler, he astonished the writer by
quoting from his work, "Alps and Sanctuaries" (p. 196), the following
passage:--
"The question of whether it is better to abide quiet, and take
advantage of opportunities that come, or to go farther afield in
search of them, is one of the oldest which living beings have to
deal with. It was on this that the first great schism or heresy
arose in what was heretofore the catholic faith of protoplasm. The
schism still lasts, and has resulted in two great sects--animals
and plants. The opinion that it is better to go in search of prey
is formulated in animals; the other--that it is better, on the
whole, to stay at home, and profit by what comes--in plants. Some
intermediate forms still record to us the long struggle during
which the schism was not yet complete.
"If I may be pardoned for pursuing this digression further, I would
say that it is the plants, and not we, who are the heretics. There
can be no question about this; we are perfectly justified,
therefore, in devouring them. Ours is the original and orthodox
belief, for protoplasm is much more animal than vegetable. It is
much more true to say that plants have descended from animals than
animals from plants. Nevertheless, like many other heretics, plants
have thriven very fairly well. There are a great many of them, and,
as regards beauty, if not wit--of a limited kind, indeed, but still
wit--it is hard to say that the animal kingdom has the advantage.
The views of plants are sadly narrow; all dissenters are
narrow-minded; but within their own bounds they know the details of
their business sufficiently well--as well as though they kept the
most nicely-balanced system of accounts to show them their
position. They are eaten, it is true; to eat them is our intolerant
and bigoted way of trying to convert them: eating is only a violent
mode of proselytizing, or converting; and we do convert them--to
good animal substance of our own way of thinking. If we have had no
trouble we say they have 'agreed' with us; if we have been unable
to make them see things from our point of view, we say they
'disagree' with us, and avoid being on more than distant terms with
them for the future. If we have helped ourselves to too much, we
say we have got more than we can 'manage.' And an animal is no
sooner dead than a plant will convert it back again. It is obvious,
however, that no schism could have been so long successful without
having a good deal to say for itself.
"Neither party has been quite consistent. Whoever is or can be?
Every extreme--every opinion carried to its logical end will prove
to be an absurdity. Plants throw out roots and boughs and leaves:
this is a kind of locomotion; and as Dr. Erasmus Darwin long since
pointed out, they do sometimes approach nearly to what is called
travelling; a man of consistent character will never look at a
bough, a root, or a tendril, without regarding it as a melancholy
and unprincipled compromise. On the other hand, many animals are
sessile; and some singularly successful genera, as spiders, are in
the main liers-in-wait."
This exquisitively written passage the writer was quite unaware of
having read, though he possessed and had perused the work quoted, nor
can he understand how such an admirable exposition could have escaped
notice. Had he read it: had he assimilated it so thoroughly as to be
unconscious of its existence; is this a case of rapid growth of
automatism? He cannot say.
To return to the main point, it would seem that specialization is
directly proportionate to activity, and when we compare the infinitely
diverse organization of the animal with the comparative simplicity of
the vegetable world, this conclusion seems to be inevitable.
[Illustration]
[9] Pflüger's Archiv. f. d. ges. Phys. Bd. xxix, 1882, quoted by
Romanes. Mental Evolution, p. 80, 1883. _Op. cit._ p. 80.
[10] Quoted by Romanes, _op. cit._ p. 81.