by Grant Allen
And now let me briefly sum up the points which I have been endeavouring to italicise in this preamble. Colour, objectively viewed, is nothing more or less than the different rate of oscillation in different æther-waves. The colour-sense, subjectively viewed, is an exaggerated difference of perception attached to the effects of these external agencies, which really differ so very little between themselves. And the problem of its origin is this — How did these slight differences in the frequencies of æther-waves reflected from various organic or inorganic bodies come to have such disproportionately diverse sensations attached to them in consciousness? In other words, when red light differs from blue light only in degree, why does red differ from blue, as we know them, in kind?
The questions thus proposed our future chapters must endeavour to answer.
CHAPTER III.
THE ORGAN OF VISION.
The perception of light is not equally valuable to all classes of animals. It seems to be specially connected with the power of locomotion. Sessile or sedentary animals, as a rule, do not possess any form of visual organ; while very free and active animals, even of lower organisation, have well-marked eyes. The Echinodermata, for example, are far more highly evolved creatures than the Medusæ, but their habits are comparatively sluggish, while the Medusæ lead a wandering, predatory life; and we find that the former class are apparently quite eyeless, while the latter have distinct ocelli, which in some cases reach a considerable complexity of structure.
Still more clearly is this connection made evident by the metamorphoses of many creatures which pass from a free to a fixed state. The young barnacles and balanids are active, locomotive animals, furnished with eyes, antennæ, and limbs; but after a period of activity, they finally fix themselves upon some solid object, and undergo a loss of all their higher sense-organs. Similar changes take place among the parasitic Entomostraca, the Tubicolar Annelids, and many Mollusca. These must be regarded as cases of degradation or retrogressive development.
Conversely, the Medusæ are shown, by their peculiar mode of development, to be the descendants of hydriform polypes. During their sessile stage, when they exactly resemble the true Hydroidea, they are as destitute of eyes as the other members of that order. But when they acquire their tentacles and assume the free mode of life, the ocelli are produced together with the other mature organs. This must be regarded as a case of progressive development.
If we examine the various classes of animals in order, the same general connection between free locomotion and vision will be forced upon us once more. Passing over the Protozoa, which of course are too humble in structure to exhibit any such complex nervous organs as eyes, and beginning with the Radiata, we see that the only class in that division which possesses high powers of locomotion is the Discophora, or jelly-fish, and this is also the only class provided with visual organs. Among the Nematophora or Echinodermata, which are all very sedentary animals, eyes are doubtfully present. The lower vermiform Articulata are mostly entozoic, and these of course are quite blind; but the few species which swim freely in water by means of cilia have eyes with distinct lenses. The free leeches have a ring of eyes around the sucking disc. The highest of these vermiform creatures — the Nereidæ, Peripatidæ, and Polyophthalmidæ — are all very locomotive, and all have very highly developed organs of vision. So likewise have the active little Rotifera. The Arthropoda, or true articulates, yield like results. Thus, among the Crustacea, the Cirrhopoda in their fixed state and the parasitic Entomostraca are sightless; but all the higher free crustaceans are provided with eyes, which in the active crab and lobster orders attain a high degree of perfection. The flying insects show us eyes of great complexity, inferior only to the same organs in vertebrates, if even to those. Yet while most of the Hymenoptera (including the wasps and bees) have very acute vision, it is noteworthy that the ants, which have practically lost their wings, are almost, and in some species quite, blind. It is also a remarkable fact that the male and female ants which are winged possess three ocelli, wanting in the wingless neuters. Among the Mollusca, in like manner, the lower molluscoid animals, most of which are fixed, have no organs of vision whatsoever; the bivalve mollusks, leading very sedentary lives, are provided only with doubtful ocelli; the relatively active univalves have true eyes, but of low organisation; while the free-swimming Cephalopods (cuttle-fish and their allies) have eyes as highly developed as those of many fishes. Lastly, the vertebrates, the most active division of any, show us the highest visual organs of all.
We shall have reason similarly to conclude hereafter that the colour-sense, the most advanced mode of vision, is specially strong amongst the flying insects, the fishes (marine analogues of flying creatures), the birds, and the very active forestine mammals. Its high development in these classes is shown as well by the part they have borne in the evolution of fruits, flowers, and coloured organisms, as by their own brilliant hues, the probable result of sexual selection.
Such a general connection between locomotion and vision is exactly what we should have expected from the nature of the case. A sessile animal, lying in wait for its food, can derive little or no benefit from the possession of visual organs. Even if it could see the approaching prey or the nearing enemy, the knowledge of their vicinity would be useless without the power of locomotion, whereby it might seize the one or avoid the other. Accordingly, most sessile animals are provided with very different organs for the prehension of food, and very different means for withdrawal from threatening danger. Some of them possess long floating arms or tentacles, spread out in every direction to catch the passing prey, which they cannot possibly secure unless it actually come within reach of their grasp. These for the most part withdraw themselves from attack into a solid tube, as in the case of the Sertularidæ, the Tubicolar Annelids, the Balanidæ, and the Bryozoa; or else curl themselves up into a contracted mass, as in the Hydra, Sea-anemones, Crinoidea, and Rotifera. Others, again, like the bivalve Mollusca, are enclosed for protection in stout shells, and obtain their food by the creation of currents in the surrounding water. A second group, that of the Entozoa, live in the interior of larger animals, often shut off from the access of light, and bathed by the nutritive fluid of their hosts. These, also, apparently find the possession of eyes no benefit to them. Accordingly, animals originally leading a life of either sort here described — sessile or parasitic — seldom or never acquire the power of vision; while animals originally possessing that power, which afterwards adopt either of these modes of life, usually or invariably lose their eyes, and become degraded in many other ways, in accordance with the Law of Parsimony, whereby all unnecessary organs become gradually obsolescent.
On the other hand, any animal which has acquired freedom of motion will naturally derive great advantage from any premonition of food or enemies in his neighbourhood. Such indications will enable him to rush upon the former or to dart away from the latter. There are various modes by which information of the sort may be given, as by those material particles which arouse the sense of smell, or those undulations of the atmospheric or aqueous medium which awaken the sense of hearing; but the waves of æther described in the last chapter form by far the most certain premonition of all approaching or neighbouring objects, and their reactions finally result in the sense of sight. Of course such a sense cannot arise amongst animals which live perpetually in the dark, like the cestoid and nematoid worms, the lob-worm, and the common earth-worm, all of whose freer relatives are provided with more or less perfect eyes; and even those animals which originally possessed visual organs lose them partially or entirely under like circumstances, as we see in the Bopyridæ, Acarina, and many other parasites, the blind moles, and the well-known sightless fish and reptiles of the Kentucky and Carinthian caverns. Similarly, most very deep-sea organisms are blind, though some remarkable exceptions occur. But amongst all the higher free locomotive and open-air or shoal-water animals we find some form of mechanism for the perception of light-waves, developed in rough proportion to the p
erfection of the motor system.
There is good reason to believe that such a mechanism has been independently evolved, time after time, by several distinct leading orders in all the great classes of animals. The eye of the bee, of the cuttle-fish, and of the eagle, have each apparently been separately developed from unlike remote sightless ancestors. Accordingly, the diversity of structure among these organs is so great, that it would obviously be impossible to give even a brief account of their leading morphological peculiarities in a single introductory chapter. It must suffice here to trace out a few of the main steps in the evolution of such organs, from the strictly psychological point of view.
Simple undifferentiated animal tissue, such as we see in the Rhizopoda, is probably more or less affected by incident æther-waves, like many other organic and inorganic substances. But in order to produce even the most vague and indeterminate sensation of light — or rather, sensation having light for its exciting cause, since the sensation itself (if any) is probably quite indefinite in quality — certain portions of the external coat must apparently be specialised by the collection of a relatively large amount of matter unusually sensitive to light, and directly connected with some simple or complex nervous centre. Such spots are always marked by the presence of pigmentary substances, which seem to play an important part in the function of sight. The simplest form in which they occur is that of the ocelli among naked-eyed Medusæ. These consist of small masses of pigment cells, surrounding a minute silicious crystal; and they are usually placed on the under edge of the umbrella-like disc. It may almost be doubted whether we can fairly attach the idea of sensation in any form to these very simple animals; but at any rate, we now know with certainty that the ocelli are organs acted upon by light, and responsive to its stimulation. Mr. G. J. Romanes, however, the latest investigator of the subject, believes that the eyes of Medusæ are the simplest possible, because the interval between the stimulus and the response is so relatively great that, were it any greater, the animal could hardly derive any advantage from the organs.
In such very rudimentary eyes, the only perception (or affection) possible is that of light or its negation, the latter being probably the most important. We may perhaps dimly figure to ourselves its nature by shutting our eyes and then passing one hand between them and the light. Some such vague consciousness (if any) of a change in the environment, is doubtless the utmost conjectural limit of discophorous vision.
The first step in progressive development from this earliest form of visual organ would consist in a simple increase in the power of distinguishing light from darkness. This step appears to be the principal one taken by the ordinary univalve Mollusca (Gasteropoda), whose eyes probably only inform them of such wide distinctions.
But an eye, to be of any special use, must also give more definite and particular information with regard to surrounding objects, and this information can best be communicated by some mechanism for the perception of form. A single percipient organ, every part of which is simultaneously and equally affected, cannot afford indications of such a sort. In order to obtain definite information as to the shape and disposition of neighbouring bodies, we must have a number of separate sensitive elements, each directed towards a point in the environing space, and subtending a greater or less angle. Every one of these elements must be provided with a nerve-fibre of its own, and connected with some percipient centre. The minuteness of discrimination must depend upon the number of such sensitive elements and the angles which they respectively subtend.
To trace out in detail the gradual steps by which such structures were evolved would be both tedious and difficult, though certain materials exist for the purpose in the simple and compound eyes of insects and their larvæ, and in the eyes of some lower vertebrates. But it will suffice for our present object to describe, in rough generalisations, the means adopted for the purpose in the most perfect eyes, such as those of bees or of mammals. Here a large number of separate nerve-terminals are arranged in a more or less semicircular form, with single or numerous lenses, which cast the æther-waves upon their percipient surfaces. Each such terminal answers to a separate point in the visual field, and the mechanism of the lens is so arranged that æther-waves from that point alone fall directly on its focal surface. Thus every point in the visual field is represented to the mind by an excitation of the corresponding terminal; and the number and position of the terminals affected gives the animal a clue to the shape and place of the object. The interpretation of these visual symbols into tactual and muscular terms becomes apparently automatic or instinctive in many cases.
In the human eye, which may be taken as a fair specimen of that found among mammals generally, the main portions of the mechanism may be thus briefly summarised. The external or optical-instrument portion consists of a viscid lens, whose shape and focus may be altered by muscular contraction, so as to converge at will æther-waves from different sources at varying distances upon a given point behind it. The internal or nervous and percipient portion consists of the retina, essentially a network of nerve-terminals, belonging to two different orders, known as rods and cones. The excitation, in varying degrees, of these terminals, gives rise to the perception of the visual field. When no part is excited, we get the blank form of visual consciousness known as darkness. When the whole is feebly excited, as through the eyelids, we get a faint general consciousness of light. When the whole is excited in the normal manner, the eyes being open and turned toward an illuminated field, we get a consciousness of mingled light and shade, yielding us indications of form. Those parts of the visual field which reflect large quantities of æther-waves yield us the sensation of relative light; those parts which reflect small quantities yield us the sensation of relative shade.
Were an eye so constituted to possess no further powers, the whole visual field would appear to it monochromatic, or rather strictly achromatic, and all objects would look as we see them in a stereoscopic picture. But the human eye is also capable of distinguishing the quality of light as well as its intensity. Not only can we discriminate between black, white, and grey, between much and little illumination, but we can also discriminate between red, blue, yellow, and green, between æther-waves of greater or less frequency. This last mode of perception — the colour-sense — is the only one with which we are concerned in the present volume; and we shall therefore leave aside all other questions of visual development as foreign to our purpose. Moreover, as our point of view is psychological rather than physiological or anatomical, we shall regard the problem of its origin solely from the mental side, without inquiring into the nature of the mechanism employed or the functions of its various parts.
Any other method of inquiry would at present be premature. Even in the human eye, where the existence of a colour-sense is certain, we know little or nothing about the mode of its production. There are good reasons, it is true, for suspecting that colour-perception is the special function of the cones, while the discrimination of light and shade is set down to the rods; both because we find colour-perception most pronounced in those parts of the retina where the cones are most thickly massed, near the central point, and less active in those parts where they are relatively few, near the periphery; and because the cones are wholly wanting in the eyes of nocturnal animals, which only require to distinguish between light and shade. But the physiology of the cones is much disputed, and the accepted theories can only be regarded as provisional. Moreover, in insects, where the colour-sense is most certain after the human species, we have not even a conjectural hypothesis of the mode in which it acts.
It may, however, be worth while, before we pass on to our proper subject — the origin and development of the colour-sense, — briefly to state the current theory as to the mechanism for the perception of colour in mankind. This theory, first proposed by Young, and adopted by Helmholtz and Schultze, supposes that each spot on the retina contains a number of nerve-terminals, each of which is capable of excitation by one colour only, or, in other words, by
æther-waves having a determinate rate of rapidity, and no others. By these terminals, a compound æther-wave would be decomposed into its constituent elements, which would arouse sensation in the corresponding nerve-fibres. It is usual to assume three such primary percipient elements, adapted respectively to the stimulation of red, green, and violet light. All other colours would be represented in consciousness by combinations of these in varying degrees of intensity. It is probable, however, that the real number of separate kinds of terminal is vastly greater. Moreover, considerable doubt hangs over the mode of excitation in the cones themselves, each of which is supplied with a large number of ultimate nerve-fibres (axis cylinders), and is therefore in all probability a compound, not a simple, percipient element. It has lately been suggested that each cone may be provided with separate portions for the perception of the various elementary colours; and these portions may be divided either longitudinally or transversely. But the whole subject is still wrapped in the greatest obscurity, viewed from the physiological side; and it is only by approaching it psychologically that we can hope to arrive for the present at any decisive result.
CHAPTER IV.
INSECTS AND FLOWERS.
Now that we have examined a few amongst the various modes by which waves of æther, as a whole, may affect the sensory system of animals, let us turn at last to our more proper subject, and inquire by what steps the different kinds of æther-waves came to be differentially cognised in consciousness as red, green, yellow, or blue. We find ourselves now face to face with the ultimate problem which we have determined, if possible, to solve in the present volume. We must see what immediate advantage animals could gain from the possession of a nervous structure capable of that differential stimulation by the diverse varieties of light which we know subjectively as the colour-sense.