In general one may say that whereas plants are an essential to almost any landscape, animals seldom affect one. There are some odd exceptions. A worm (Tubifex rivulorum), belonging to the same class (Oligochaeta) as the common earthworm, sometimes lives in such abundance in streams as to give a red colour which the artist must acknowledge with his brush. Vast swarms of Noctiluca (a Protozoon so small as scarcely to be visible to the naked eye) and other phosphorescent animals may light up the ocean at midnight. Millions of guillemots may change the aspect of sea cliffs almost as much as a covering of vegetation. But far more than any other animal the coral has impressed itself upon the scenery of the world. The reefs it builds up put to shame as insignificant the greatest marine barriers of mankind. The whole of many men’s existence is spent upon circlets of calcium carbonate in the middle of the Pacific and other oceans, which would never have existed but for this laborious little creature.