632 THE POPULAR SCIENCE MONTHLY. 



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 disap- 

 pear 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 un- 

 broken series, from the temporary spots provoked by light or 

 electricity to the permanent markings of animals.* 



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 larvae and pupse of both the small and the 

 large cabbage butterfly. He kept the larvse during their meta- 

 morphoses in boxes lined with paper of different colors, and he 

 found that the colors assumed by the pupse more or less corre- 

 sponded 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 con- 

 firmed again by W. Petersen, f 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 rep- 

 resenting the powers of the various parts of the spectrum for 

 preventing the formation of a dark pigment, the curve has its 



* See the polemics engaged upon this subject in Biologisches Centralblatt, December 

 15, 1890, and July 15, 1891 ; as also the Zoologische Jabrbucher, 1891. 



f Zur Frage der Chromophotographie bei Schmetterlingen, in Sitzungsberichte der Dor- 

 pater Naturforscher-Gesellschaft, 1890, vol. x, p. 232. 



