This little book is only a sketch of what its Author desired it to be, and he never saw the completed manuscript. Beginning with the fundamental idea that decoration is based upon structure, he saw that this was due to the fact that in the lower, transparent, animals, colour is applied directly to the organs, and that the decoration of opaque animals is carried out on the same principle— the primitive idea being maintained. Where function changes the pattern alters, where function is localized colour is concentrated : and thus the law of emphasis was evolved. Symmetry was a necessary consequence, for like parts were decorated alike, and this symmetry was carried out in detail apparently for the sake of beauty, as in the spiracular markings of many larva?. Hence the reason for recognizing the law of repetition.