Walls noted that many diurnal mammals had large corneas, fewer types of photoreceptor cells to see colors, and a tapetum lucidum—a reflective surface that increases light absorption by the retina.
Walls 指出,许多昼夜活动哺乳动物都有大角膜、较少类型感光细胞来识别颜色,以及绒毡层——一种增加视网膜对光吸收反射表面。