We have thus numerous examples of colored epigæan animals bleaching in caves, and also bleached cave animals turning dark when exposed to the light. We have also animals in which the side habitually turned to the dark is colorless, while the side habitually turned to the light is colored. Finally, we have cave animals that are permanently bleached.
Natural selection can not have affected the coloration of the cave forms, for it can be absolutely of no consequence whether a cave species is white or black. It could affect the coloration only indirectly in one of two ways: First, as a matter of economy, but since the individual is in part bleached by the direct effect of the darkness there is no reason why natural selection should come into play at all in reducing the pigment as a matter of economy; second, Romanes has supposed that the color decreases through the selection of correlated structures—a supposition he found scarcely conceivable when the variety of animals showing the bleached condition is considered.
Fig. 4 a.
Fig. 4 b.
Fig. 4 c.
Fig. 4 a, b, c.—Three views of Amblyopsis.
Panmixia can not account for the reduction of the color, since it returns in some species when they are exposed to the light, and disappears to a certain extent in others when kept in the dark. Panmixia, Romanes thinks, may have helped to discharge the color. In many instances the coloration is a protective adaptation, and therefore maintained by selection. Panmixia might in such instances lower the general average to what has been termed the “birth mean.” Proteus is perhaps such an instance. But in this species the bleached condition has not yet been hereditarily established, and since each individual is independently affected “the main cause of change must have been of that direct order which we understand by the term climatic.”