In the process of evolution, frogs climbed out of the water on to the land, but they continued to rely mainly on breathing through their skin. Except for toads, whose skin retains water better, most frogs need to keep their skin wet and avoid ultraviolet rays, which is why frogs prefer dark, wet places.
在生物演化過程中，青蛙從水中爬上陸地，主要仍依賴皮膚呼吸，除了蟾蜍皮膚因為角質化，水分不易流失外，大部分青蛙必須保持皮膚濕潤、避免紫外線照射，因此青蛙喜歡躲在陰濕處。