Let us first take the cases of abnormal marking as shown in disease. An ordinary rash, as in measles, begins as a set of minute red spots, and the same is the case with small pox, the pustules of which sometimes run together, and becoming confluent form bars, which again enlarging meet and produce a blotch or area abnormally marked. It was these well-known facts that induced us to re-examine this question. Colouration and discolouration arise from the presence or absence of pigment in cells, and thus having, as it were independent sources, we should expect colour first to appear in spots. We have already stated, and shall more fully show in the sequel, how colouration follows structure, and would here merely remark that it seems as if any peculiarity of structure, or intensified function modifying structure, has a direct tendency to influence colour. Thus in the disease known as frontal herpes, as pointed out to us by Mr. Bland Sutton, of the Middlesex Hospital, the affection is characterized by an eruption on the skin corresponding exactly to the distribution of the ophthalmic division of the fifth cranial nerve, mapping out all its little branches, even to the one which goes to the tip of the nose. Mr. Hutchinson, F.R.S., the President of the Pathological Society, who first described this disease, has favoured us with another striking illustration of the regional distribution of the colour effects of herpes. In this case decolouration has taken place. The patient was a Hindoo, and upon his brown skin the pigment has been destroyed in the arm along the course of the ulnar nerve, with its branches along both sides of one finger and the half of another. In the leg the sciatic and saphenous nerves are partly mapped out, giving to the patient the appearance of an anatomical diagram.[14] In these cases we have three very important facts determined. First the broad fact that decolouration and colouration in some cases certainly follow structure; second, that the effect begins as spots; thirdly, that the spots eventually coalesce into bands and blotches. In birds and insects we have the best means of studying these phenomena, and we will now proceed to illustrate the case more fully. The facts seem to justify us in considering that starting with a spot we may obtain, according to the development, either an ocellus, a stripe or bar, or a blotch, and that between, these may have any number of intermediate varieties. * * * * *