The Influence of External Conditions 13 
A number of Arctic animals become white in winter. This 
change seems to be in part due directly to the cold, for it has 
been found if these animals are transferred to warmer climates 
they show less marked changes on the approach of winter. 
Flounders and some other fish and some amphibians become 
lighter in color on a light background and darker on a dark 
background. The most remarkable case of this sort is that of 
the pupe of certain butterflies. If the pupation takes place on a 
light background, the chrysalidsare lighter; andif ona dark back- 
ground, they are darker. Experiments by Poulton ' have shown 
that this effect is produced directly through the skin and not 
through the ocelli. Poulton thought in one case that even the 
color of the silk in which the caterpillar incloses itself is influ- 
enced by the color of the background, but this has been shown 
not to be the case. 
It is popularly supposed that the African chameleon becomes 
green in green surroundings, and brown in a dark environment, 
but this is probably not true; at least ithas been shown in another 
lizard, Anolis, that can also change from green to brown and 
the reverse, that the animal is as a rule green if warm and 
brown if cold. The effects are produced by change in the pig- 
ment of the dermal pigment cells. In the cold the black pigment 
spreads out over the surface and conceals the stationary green 
pigment. In the warmth the black pigment migrating inward 
exposes the green. Light has a somewhat different effect. In 
both the African chameleon and in Anolis a strong light acts 
like a low temperature, causing the black pigment to migrate 
to the surface, and a faint light or darkness acts like a high 
temperature, causing the black pigment to wander inward, 
so that the animal becomes green. Other lizards give reverse 
effects in light. Parker and Starratt have shown for Anolis that 
when both light and heat act together, the results are as follows: 
‘ For details of the experiment, see Poulton, “The Colours of Animals,” 
p. 110, 1890; ‘(Further Experiment. upon the Colour Relation,” etc., Trans. 
Ent. Soc., p. 293, London, 1893; “An Inquiry into the Cause and Extent of 
a Special Colour Relation,” Proc. Roy. Soc., Vol. XII, 1887. 
