That gaudy colors have appeared or been present in animals that are protected in other ways is not improbable, when we consider the rôle that color plays everywhere in nature. That the presence of such colors may, to a certain limited extent, protect its possessor may be admitted without in any degree supposing that natural selection has directed the evolution of such color, or that it has been acquired through a life and death struggle of the individuals of the species. Sexual Dimorphism[29] and Trimorphism Footnote 29: This term is used here in the sense employed by Darwin. The same term is sometimes used for those cases in which the male departs very greatly from the female in form. It has been found in a few species of animals and plants that two or more forms of one sex may exist, and here we find a condition that appears to be far more readily explained on the mutation theory than on any other. The most important cases, perhaps, are those in plants, but there are also similar cases known amongst animals, and these will be given first. There is a North American butterfly, _Papilio turnus_, that appears under at least two forms. In the eastern United States the male has yellow wings with black stripes. There are two kinds of females, one of which resembles the male except that she has also an orange “eye-spot”; the other female is much blacker, and this variety is found particularly in the south and west. The species is dimorphic, therefore, mainly in the latter regions. The cases of seasonal dimorphism offer somewhat similar illustrations. The European butterfly, _Vanessa levana-prorsa_, has a spring generation (_levana_) with a yellow and black pattern on the upper surface of the wings. The summer generation (_prorsa_) has black wings “with a broad white transverse band, and delicate yellow lines running parallel to the margins.” These two types are sharply separated, and their differences in color do not appear to be associated with any special protection that it confers on the bearer. These facts in regard to Vanessa seem to indicate that differences may arise that are perfectly well marked and sharply defined, which yet appear to be without any useful significance. We meet with cases in which the same animal has at different times of year different colors, as seen in the summer and winter plumage of the ptarmigan. There is no direct evidence to show how this seasonable change has been brought about; but from the facts in regard to Vanessa we can see that it might have been at least possible for the white winter plumage, for instance, to have appeared without respect to any advantage it conferred on the animal, but after it had appeared it may have been to a certain degree useful to its possessor. [Illustration: