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.