Countershading: Universally deceptive or deceptively universal?

The presence of dark dorsa and light ven tra on animals is often attributed to an adaptive effect of shape-obliteration when illumination is from above. In fact, direct evidence for this effect is scant. The various meanings of the term 'countershade' have been conflated, and alternative explanations for the evolution of the trait have been given less attention. The phenomenon is much less likely to benefit terrestrial species than aquatic ones. Further experimental investigations of the idea are needed.