648 The Colors of Animals and Plants. [ November, 
oblige us to accept them as actual phenomena. It is a kind of 
natural photography, the particular colored rays to which the 
fresh pupa is exposed in its soft, semi-transparent condition ef- 
fecting such a chemical change in the organic juices as to pro- 
duce the same tint in the hardened skin. It is interesting, how- 
ever, to note that the range of color that can be acquired seems 
to be limited to those of natural objects to which the pupa is 
likely to be attached; for when Mrs. Barber surrounded one of 
the caterpillars with a piece of scarlet cloth no change of color 
at all was produced, the pupa being of the usual green tint, but 
the small red spots with which it is marked were brighter than 
usual. 
In these caterpillars and pup, as well as in the great majority 
„of cases in which a change of color occurs in animals, the action 
is quite involuntary ; but among some of the higher animals the 
color of the integument can be modified at the will of the animal, or, 
at all events, by a reflex action-dependent on sensation. The most 
remarkable case of this kind occurs with the chameleon, which has 
the power of changing its color from dull white to a variety of 
tints. This singular power has been traced to two layers of pig- 
ment deeply seated in the skin, from which minute tubes or 
capillary vessels rise to the surface. The pigment layers are 
bluish and yellowish, and by the pressure of suitable muscles 
these can be forced upward either together or separately. When 
no pressure is exerted the color is dirty white, which changes w 
various tints of bluish, green, yellow, or brown, as more or less 
of either pigment is forced up and rendered visible. The animal 
is excessively sluggish and defenseless, and its power of chang- 
, ing its color to harmonize with surrounding objects is essential 
to its existence. Here, too, as with the pupa of P apilio Nireus ‘ 
colors such as scarlet or blue, which do not occur in the immedi- 
ate environment of the animal, cannot be produced. Somewhat 
similar changes of color occur in some prawns and flat-fish, — 
cording to the color of the bottom on which they rest. This 2 
very striking in the chameleon shrimp (Mysis chameleon), 
which is gray when on sand, but brown or green when among 
sea-weed of these two colors. Experiment shows, however, 
when blinded the change does not occur, so that here, to, WF 
probably have a voluntary or reflex sense-action. Many pe 
are known among insects in which the same species has & i p 
ent tint according to its surroundings, this being particulary 
marked in some South African locusts which correspond 
