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Poor little May-flies on the pools of Venlake, you have at best but a hard life of it! Though your wings are fairy-like and light as gauze, though the sunshine plays upon your dancing bodies with opalescent hues, though you spend your time merrily enough to all seeming in flitting and flirting by the cool rivulet, yet is your appointed span but twenty-four hours long, and even for that short space your courtship and your maternity is environed with manifold dangers and endless foes. You pass your days between the Scylla of sunshine and the Charybdis of cloudy skies. When the sun shone yesterday, you were devoured in the midst of your love-making by the gay swallows; when the clouds cover the heaven to-day, I see the trout are leaping to engulf you as you try in vain to lay your eggs in peace and quiet on the calm surface of the water. The fish can see you quite enough against the background canopy of cloud, and there is nothing they love better for their morning meal than a good fat mother May-fly.

I wonder very much what thoughts pass through the heads of these jumping trout as they gaze up eagerly towards the vast white sheet above them, just dappled here and there by the little spot of darkness that forms to them the visible symbol of an eatable insect. One of the great dangers, indeed, which surround the path of scientific psychology is that of being too exclusively human. Here more than anywhere else in science the old Greek doctrine that man is the measure of all things seems especially to beset us on every side. Our own consciousness being the only consciousness which we can experimentally examine, we are peculiarly liable to accept its component elements as being the component elements of all other consciousness whatsoever. It is very hard—some philosophers have even told us it is impossible—to construct a comparative psychology, as we can construct a comparative osteology or a comparative philology. All the other minds about which we can obtain even the second-hand information given us by language are still human minds; and for the animal consciousness generally we are reduced to very inferential and doubtful data.

Yet even here a good deal can be done by careful sifting of facts, if only we know what facts to sift. The general principle of nihil est in intellectu stands us in good stead when once we have been able to discover what was before in sensu; and this we can often do provided we take the trouble to follow out all the hints supplied us by the nervous system and by the habits or peculiarities of animals. In some fishes, for instance, there is every indication of the preponderance of smell over sight as an intellectual and guiding sense. In the sharks and rays the membrane of the nose is enormously developed; the olfactory nerve is by far the largest and most important in the body; the central organs directly or indirectly connected with it form the main mass of the brain; and the indications of habit, as well as the sniffing-muscles attached to the nostrils, all go to show that smell is really the chief sense-endowment of these predatory species. On the other hand, their eyes are relatively small and poorly developed, their optic nerves and lobes are unimportant, and the general indications (about which it is only possible here to speak negatively) do not lead one to suppose that sight is a sense of much practical value to the sharks and rays. There are other classes of fish, however, in which sight seems to play a far more important part, and here it is perhaps possible to institute some rough comparison as to relative perfection with the case of the human eye and brain.

The class of fish in which the eye is apparently best developed is that of the teleosteans, to which belong the perch, salmon, cod, sole, turbot, and generally speaking almost all the best-known and edible species, including the trout of Venlake. These fish are comparatively late arrivals in our oceans and rivers, when we judge by a geological standard; but they have rapidly lived down the great ganoids which preceded them, and have reduced the shark family and the lampreys to a few predatory or parasitic species. Externally and structurally they differ in many particulars from all the other classes of fish, which are now represented only by a relatively small number of survivors; but on the psychological side they differ most conspicuously in this particular—that, while the remaining ganoids, sharks, and lampreys all show signs of depending mainly upon smell, their modern superseders show signs of depending mainly upon sight. The eye of these fishes is large and fairly developed; the optic nerves are big, and arranged in the same manner as among the higher animals; and the optic centres form by far the largest portion of the brain. On the other hand, the olfactory nerves and centres are small and shrivelled. The indications of habit are certainly rather inferential, yet they all point in the same direction as these structural facts.

Most common fish certainly find their food mainly by means of sight. The careful way in which it is necessary to imitate flies in order to deceive the wary trout shows that they can pretty accurately distinguish forms and colours. The rapidity and certainty with which other fish will rise to an artificial minnow on a trolling-line sufficiently proves the rapidity of their perceptions. The imitative devices or mimicry which exist among many species similarly prove how sharp are the eyes of their enemies; for these resemblances can only have been developed in order to deceive the senses of other fishes, and would not, of course, go beyond the point at which they proved useful to the species. All flatfish closely imitate the colours and arrangement of the sand or pebbles on which they lie: and it is often difficult even for a human eye to detect a sole or a flounder in an aquarium, although one may be perfectly sure that it is to be found at the bottom of a particular tank. Some of them have special pigment cells, like those of the chameleon, which they squeeze out in varying proportions till they exactly resemble their surroundings; and as this action ceases when the fish is blind, it shows that the protected fish themselves, as well as their enemies, are conscious of minute differences in form and colour. All the animals which inhabit the sargasso weed are also coloured exactly like it, and so closely imitate it in many ways that I have often narrowly examined a piece of the weed freshly brought up in a bucket, and yet failed to detect any sign of life till I lifted the spray from the water and so compelled the hide-aways to reveal themselves. There is one pipe-fish, indeed, from the Australian coasts, which so exactly mimics the fucus in which it lurks that nobody would believe it is a fish rather than a branch of the weed round which it curls, until he has dissected it. The necessity for such close resemblances is the best possible proof of acute sight in fishes exactly analogous to our own.

That this faculty of vision includes a perception of colour as well as form is shown by the same facts; but there are other facts which seem to indicate it yet more clearly. The teleosteans, which possess these developed eyes and optic centres, are the only fish in which Mr. Darwin has noted the occurrence of ornamental colours or appendages, due, as he believes, to selective preferences on the part of the animals themselves. It is curious, too, that all the indirect proofs of colour-sense in fishes occur among this same group. The ornamental colours generally coexist with very excitable tempers, as is also the case with such higher animals as the mandrill, the peacock, and the humming-birds; and in the little fighting-fish kept as pets by the Siamese, the brilliant hues are only displayed on the appearance of a rival or of the fish’s own reflection in a mirror. The moment the little creature sees another of his own kind, he exhibits all his colouring, and rushes against his enemy covered with metallic tints, and waving his projected gills like the wattles of a turkey-cock. Almost all the most beautifully coloured fish are coral-feeders, dwelling among the reefs and feeding off the bright polypes and other beautiful creatures which abound in tropical seas.

This case is again quite paralleled by that of birds and insects; for the most gaily coloured species, like the butterflies, rose-beetles, humming-birds, parrots, loris, and toucans, are flower-feeders or fruit-eaters; and we may well suppose that in every case a taste for colour has been aroused in the creatures themselves during their constant intercourse with brilliant surroundings and their continual quest for brilliant kinds of food. There seems to be, in fact, a regular gradation of colour-sense and colour-beauty in fishes, the most highly perceptive being themselves apparently the most ornamented. There is also a similar gradation of general sight-faculties: from the case of a tropical shore-fish which can thrust its moveable eyes out of their sockets, and which hunts crustaceans out of water on mud-flats at ebb tide, or of an open-sea fish which swims half above the surface, and has its eyes divided horizontally into two portions, one adapted for vision in air and the other in water—to the blind fishes of the Mammoth Cave and of the marine abysses revealed to us by the explorers in the Challenger. From all these converging indications it is perhaps possible to make a nearer guess at the visual faculties of fishes than most people would be at first sight inclined to suspect.