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THE POPULAR SCIENCE MONTHLY.— SUPPLEMENT. 



gradual development of color has not been proved 

 to be the result of light. We have reared up 

 caterpillars in perfect darkness, and have found 

 their colors on reaching maturity no less brilliant 

 than those of their fellows which had been ex- 

 posed to light in the ordinary course of Nature. 

 In the case of interference-colors a change in the 

 physical condition of the integuments, conse- 

 quent of their drying and hardening on exposure 

 to the air, is doubtless necessary for their devel- 

 opment. The evolution of the pigment-colors we 

 are at present investigating, and believe that it is 

 simply due to a process of oxidation. 



Some other of the phenomena advanced in 

 support of the "light-theory" of organic color- 

 ation may also be, with great probability, referred 

 to other causes. Thus some ascribe to light the 

 fact that the upper surface of most animals is 

 more intensely colored than their under side. 

 Many fishes have a dark back, and a pale grayish 

 blue or greenish belly ; but, as Mr. Wallace points 

 out, this arrangement seems more protective than 

 due to the action of light. An enemy — say a sea- 

 bird — looking down from above will have diffi- 

 culty in distinguishing the dark back of the fish 

 amid the water. On the other hand, an enemy 

 looking from below will see the pale belly of the 

 fish against the dull bluish color of the sky as 

 seen on looking up through the water, and will 

 scarcely detect its presence. Now, were this ar- 

 rangement of colors reversed, the fish would be 

 much more readily seen, either from above or 

 from below, and the chances of its escaping from 

 its enemies would be much reduced. At the same 

 time it must be confessed that this explanation is 

 not admissible in all cases of a similar arrange- 

 ment of color. Thus in many crustaceans unable 

 to swim, and therefore not liable to be seen by 

 any enemy from below, the under surface is much 

 paler than the back. Similarly slugs — whether 

 creeping upon the ground, upon the stems or 

 leaves of vegetables — are liable to be espied from 

 the back or the sides, but never from beneath ; 

 yet in most cases their under surface is decided- 

 ly paler than their back. These instances, and 

 others which might be adduced, certainly seem 

 to agree better witli the supposition of a darken- 

 ing influence due to the freer action of light upon 

 the upper side than with that of a protective dis- 

 tribution of color. 



But from the whole of the evidence before us, 

 especial attention being paid to the case of the 

 deep-sea anemones, we are forced to conclude that 

 the coloration of an animal species is not, in the 

 mathematical use of the word, a function of the 



amount of solar radiations to which it is ex- 

 posed. That this conclusion does not compel us 

 to deny the influence of sunlight upon the hues of 

 all animals, under all conceivable circumstances, 

 scarcely needs to be stated. 



The fact that Lepidopterous larvae are in a 

 majority of cases, partially at least, of a green 

 color, is not inexplicable. They retain in their 

 bodies, in an undecomposed state, the chlorophyll 

 of the leaves upon which they have fed. Larvae, 

 on the other hand, whether Lepidopterous or Co- 

 leopterous, which feed not upon leaves, but upon 

 wood, roots, seeds, etc., not containing chloro- 

 phyll, may naturally be found deficient in this 

 green color, without our taking the presence or 

 absence of light into account. Hence we need 

 not wonder that the caterpillars of the goat-moth 

 and the wood-leopard, the larvae of the Longi- 

 cornes, Buprestidae, Dynastidae, etc., are not green : 

 they have not been consuming a green pigment. 

 But why have we comparatively so few green but- 

 terflies and moths, and so many green birds and 

 green beetles ? The green colors found in birds 

 and in beetles — with the exception of such forms 

 as Cassida, a leaf-feeder — are due not to a pig- 

 ment, but to the interference of light, so that their 

 formation must be explained on different princi- 

 ples. The paucity of green butterflies may, per- 

 haps, be traced to the fact that chlorophyll is a 

 mixture of two coloring principles l — cyanophyll, 

 which is blue, and xanthophvll, which is yellow — 

 the latter of these colors being much more per- 

 manent that the other. Hence if, as appears ex- 

 ceedingly probable, chlorophyll is assimilated by 

 leaf-eating insects, a number of phenomena con- 

 nected with their coloration become at once intel- 

 ligible. We have mentioned in an earlier part of 

 this paper that among animal tints pigment-greens 

 are generally the first to fade, and that they be- 

 come a dull yellow or a yellowish drab, as may 

 be observed in a specimen, say, of Cassida eques- 



i tris, which, however carefully preserved, soon 

 loses its pale-green hue, and turns yellowish. The 

 reason of this change, we contend, is that the cy- 

 anophyll or blue coloring-matter first undergoes 

 decomposition, while the yellow xanthophyll alone 

 remains. A similar change, taking place in the 

 living animal in its pupa condition, is the cause 



I why pigment-greens are so rare alike among Lep- 



i 



1 Some authorities consider that chlorophyll is a 



i mixture not of two, but of three coloring principles. 



' (Freray and Cloez), or of four (Stokes). As these, 

 however, are in all cases found to be respectively blue 

 and yellow, the view vr e have taken will not be affected 

 by these discordant results.. 



