Page:Popular Science Monthly Volume 43.djvu/650

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character, and have the same origin, as the also temporary and irregular spots which appear in other fishes, as well as in several tritons and many Gekonides, without the interference of man. Some of the provoked changes of color do not entirely vanish after the irritation is over, and they belong to the same category as the spots which appear in many animals in youth, and disappear with growing age. Moreover, it is maintained that a series of slow gradations may be established between the irregular spots, the spots arranged in rays, and finally the stripes, such as we see them in higher mammals like the zebra or the tiger; and if these generalizations prove to be correct, we shall thus have an unbroken series, from the temporary spots provoked by light or electricity to the permanent markings of animals.[1]

And, finally, attempts are being made to explain some of the wonderful so-called adaptive colors of insects as a direct product of environment. Some time ago (in 1867) T. W. Wood published experiments upon the larvæ and pupæ of both the small and the large cabbage butterfly. He kept the larvæ during their metamorphoses in boxes lined with paper of different colors, and he found that the colors assumed by the pupae more or less corresponded to their surroundings. Later on E. B. Poulton made a wider series of analogous experiments, and he saw that the change of color is accomplished during the first hours when the larva spins its web; he came to the conclusion that it depends upon a certain physiological action which is transmitted to the nervous system, not only through the visual organs, but through the whole surface of the skin. These facts have now been fully confirmed again by W. Petersen,[2] but his explanation is of a more mechanical character. He maintains that the color of the pupa depends upon the pigment contained in both its cuticle and hypodermis. The pigment of the latter is green in the larva, and sometimes it remains green during the pupal stage; but it may be visible or not, according to the amount of dark pigment which is formed in the cuticle, and the amount of this dark pigment entirely depends upon the color of the light. Yellow and orange light prevents the formation of the dark pigment, and in such cases the cuticle, which remains transparent, shows the green pigment of the hypodermis. But the less bright parts of the spectrum have not the same power, and if we trace a curve representing the powers of the various parts of the spectrum for preventing the formation of a dark pigment, the curve has its

  1. See the polemics engaged upon this subject in Biologisches Centralblatt, December 15, 1890, and July 15, 1891; as also the Zoologische Jahrbücher, 1891.
  2. Zur Frage der Chromophotographie bei Schmetterlingen, in Sitzungsberichte der Dorpater Naturforscher-Gesellschaft, 1890, vol. x, p. 232.