We have to bear this consideration in mind when we take into account the susceptibilities of animals, especially of the higher animals, to red. The color sense, it is well known, is widely diffused among animals; indeed this fact has been brought forward, especially by Pouchet, to prove that there can have been no color evolution in man; this it can scarcely be said to show, since evolution does not run in a straight line, and it is quite conceivable and even probable that the ancestors of man were less dependent than many lower animals, for the means of living, on a highly developed color sense. Thus a color sense that among some creatures is so highly developed as to include even the ultra-violet rays, was among our own ape-like ancestors either never developed or partially lost. Graber, in his important investigation into the color sense of animals, showed that of fifty animals studied by him forty showed strong color preferences in their places of abode. In general he found, without being able to explain the fact, that animals which prefer the dark are red lovers, those which prefer the light are blue lovers. The common worm, with head and tail cut off, still preferred red to blue nearly as much as when uninjured. (This would seem to indicate the same kind of susceptibility to unaccustomed violet rays which we have already encountered in the phenomenon of sun-burn.) The triton and cochineal, with eyes removed and heads covered with wax, still had delicate sense for color and brightness. The flea infesting the dog had a finer color sense than the bee, while nearly all the animals Graber investigated were more or less sensitive to the ultra-red rays.