The Influence of External Conditions \ 3 



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 pupae of certain butterflies. If the pupation takes place on a 

 light background, the chrysalids are lighter ; and if on a dark back- 

 ground, they are darker. Experiments by Poulton 1 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 it has 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 : 



1 For details of the experiment, see Poulton, "The Colours of Animals," 

 p. no, 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. 



