Page:The Zoologist, 4th series, vol 5 (1901).djvu/359

Rh and the mimicking creature may possess the unison we see, but under different characters and under different conditions. Thus to a colour-blind person who visualizes blue as green, what we should understand as a wonderful resemblance in a blue animal to its blue environment would be to him the assimilation in colour of two green objects. To a near-sighted person, the mimicking resemblance of a Phasma to the leaf or twig on which it was found would probably be much greater than that appreciated by the possessor of stronger and more penetrating powers of vision; and the same fact as observed by both would, if analytically recorded in each case, be capable of modifying or enlarging our conceptions of the phenomena or theory under consideration. But how much more cogent is this suggestion if we compare the resultant of human power of vision with that possessed by other animals—say, as low in the scale of derivation as insects—whose eyes have a structure so dissimilar to our own, and whose sensory impressions are therefore likely to be so totally diverse. The very essence of the theory of evolution predicates a vast difference in the sensation of vision, which must vary as the organ does in structure. As Darwin observes:—"Within the highest division of the animal kingdom, namely, the Vertebrata, we can start from an eye so simple, that it consists, as in the Lancelet, of a little sack of transparent skin, furnished with a nerve, and lined with pigment, but destitute of any other apparatus. In fishes and reptiles, as Owen has remarked, 'the range of gradations of dioptric structures is very great.'" Wal-