40 Colouration in Animals and Plants. 



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 charac- 

 terized 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.* 



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. 



Among the butterflies we have numerous examples of the de- 

 velopment from spots, as illustrated in plates. A good example is 

 seen in our common English Brimstone (Gonepteryx rhamni) Fig. 2, 

 Plate III. In this insect the male (figured) is of a uniform sulphur 



* See photographs in Hutchinson's Illustrations of Clinical Surgery. 



