These facts are of exceeding interest. They seem to show that for these birds the retinal explosives are not the same as for us. They are R., O., and G. Moreover, the colour-globules will have the effect of excluding the phenomena of overlapping. For each kind of cone the spectrum must be limited to the narrow spectral band transmissible through the associated colour-globule. If these facts be so, it is not too much to say that the colour-vision of birds must be so utterly different from that of human beings, that, being human beings, we are and must remain unable to conceive its nature. The factors being different, and the blending of the factors by overlap being, by specially developed structures, lessened or excluded, the whole set of resulting phenomena must be different from ours. And this is a fact of the utmost importance when we consider the phenomena of sexual selection among birds, and those theories of coloration in insects which involve a colour-sense in birds.
Concerning the sense of sight in reptiles and in amphibians, little need here be said. At near distances some of them undoubtedly have great accuracy of vision. This is, perhaps, best seen in the chamæleon. In this curious animal the eyes are conical, and each moves freely, independently of the other. The eyelids encase the organ, except for a minute opening, looking like a small ink-spot at the blunted apex of the cone. The animal catches the insects on which it feeds by darting on to them its long elastic tongue and slinging them back into the mouth, glued to its sticky tip. Its aim is unerring, but it never strikes until both eyes come to rest on the prey, and great accuracy of vision must accompany the great accuracy of aim. Frogs and toads capture their prey in a somewhat similar way; and a great number of reptiles and amphibians are absolutely dependent for their subsistence on the acuteness and accuracy of their vision, which is, however, on the whole, markedly inferior to that of birds.
In fishes, from their aquatic habit, the lens and dioptric apparatus are specially modified, in accordance with the denser medium in which they live; and one curious fish, the Surinam sprat, is stated to have the upper part of the lens suited for aerial, and the lower part for aquatic vision.
Mr. Bateson[FI] has made some interesting observations on the sense of sight in fishes. He finds that in the great majority of fishes the shape and size of the pupil do not alter materially in accordance with the intensity of the light. The chief exceptions are among the Elasmobranchs (dog-fishes and skates). In the torpedo the lower limb of the iris rises so as almost to close the pupil, leaving a horizontal slit at the upper part of the eye. In the rough dog-fish, the angel-fish, and the nurse-hound, the pupil closes by day, forming merely an oblique slit. In the skate a fern-like process descends from the upper limb of the iris. The contraction in these cases does not seem to take place rapidly as in land vertebrates, but slowly and gradually.
Among diurnal fishes belonging to the group of the bony fishes (Teleosteans), the turbot, the brill, and the weever have a semicircular flap from the upper edge of the iris, which partially covers the pupil by day, but is almost wholly retracted at night.
None of the fishes observed by Mr. Bateson appears to distinguish food (worms) at a greater horizontal distance than about four feet, and for most of them the vertical limit seemed to be about three feet; but the plaice at the bottom of the tank perceived worms when at the surface of the water, being about five feet above them. Most of them exhibited little power of seeing an object below them. But though the distance of clear vision seems to be so short for small objects in the water, many of these fish (plaice, mullet, bream) notice a man on the other side of the room, distant about fifteen feet from the window of the tank. The sight of some fishes, such as the wrasses (Labridæ), is admirably adapted for vision at very close quarters. "I have often seen," says Mr. Bateson, "a large wrasse search the sand for shrimps, turning sideways, and looking with either eye independently, like a chamæleon. Its vision is so good that it can see a shrimp with certainty when the whole body is buried in grey sand excepting the antennæ and antenna-plates. It should be borne in mind that, if the sand be fine, a shrimp will bury itself absolutely, digging with its swimmerets, kicking the sand forwards with its chelæ, finally raking the sand over its back, and gently levelling it with its antennæ; but if the least bit be exposed, the wrasses will find it in spite of its protective coloration."
Fig. 34.—Pineal eye.
Modified eye-scale of a small lizard, Varanus benekalensis. (After Baldwin Spencer.)