That some such grouping or gathering together, allied to segregation, does take place, a study of spots, and especially of eye-spots, renders probable. What the nature of the process is we do not know, nor is it easy to imagine. But let us suppose a surface uniformly tinted brown. Then, if we gather some of the colouring matter into a dark spot we shall naturally leave a lighter area around it, just as we see in all our Browns and Ringlets. In this way we can see how a ring-spot can be formed. To make it a true eye-spot, with a light centre, we must also suppose a pushing away of the colour from that centre. A study of ocelli naturally suggests such a process, which is analogous to the banding of agates, and all concentric nodules. Darwin, struck with this, seems to adopt it as a fact, for he says, "Appearances strongly favour the belief that, on the one hand, a dark spot is often formed by the colouring matter being drawn towards a central point from a surrounding zone, which is thus rendered lighter. And, on the other hand, a white spot is often formed by the colour being driven away from a central point, so that it accumulates in a surrounding darker zone."[18] The analogy between ocelli and concretions may be a real one. At any rate beautiful ocelli of all sizes can be seen forming in many iron-stained sand-stones. The growth of ocelli may thus be a mechanical process adapted by the creature for decorative purposes, but the artistic colouring of many eye-spots implies greater effort. There is, however, one set of colour lines in birds and insects that do not seem to arise from spots in the ordinary way. These are the coloured feather-shafts of birds, and the coloured nerves or veins in a butterfly's wing, In these the colour has a tendency to flow all along the structure in lines. _Conclusion._ The results arrived at in this chapter may be thus summarised:-- Spots, ocelli, stripes, loops, and patches may be, and nearly always are, developed from more or less irregular spots. This is shown both by the study of normal colouring, or by abnormal colouring, or decolouring in disease. Even the celebrated case of the Argus Pheasant shows that the bands from which the ocelli are developed arose from spots. [Illustration] [11] Descent of Man, vol. ii., p. 132. [12] Quart. Journ. Sci., July 1868, p. 325. [13] Studies in the Theory of Descent. [14] See photographs in Hutchinson's Illustrations of Clinical Surgery. [15] See Plate IV. [16] Desc. Man, vol. ii, p. 133, fig. 52. [17] Compare his figs. 56 to 58 op. cit. [18] Desc. Man, vol. ii., p. 134. CHAPTER VII. COLOURATION IN THE INVERTEBRATA.