292 MERISTIC VARIATION. [part I. 



on the upper surface of the hind wings of the Wall (P. megcera) 

 are an excellent illustration of these principles of Variation. 



The principle here stated, though generally followed, is not 

 absolutely universal, and in other instances it occasionally happens 

 that even when of very minute size an eye-spot still retains all its 

 bands ; but the statement that the order of disappearance is from 

 the centre outwards and not the reverse is substantially true. 

 Some have expressed a belief that ocelli arise by the breaking up 

 of bands of colour, but this view finds no support in the facts of 

 Variation so far as the simple ocelli of such forms as Morplio and 

 the Satyridse are concerned ; for in its rudimentary condition a 

 circular eye-spot is in them a circular eye-spot still. 



The fact just stated, that in the reduction of a circular ocellus 

 its central parts are the tirst to disappear, recalls phenomena seen 

 in many cases of disturbance propagated from a centre through a 

 homogeneous medium. A whole eye-spot may come, or it may go (as 

 seen in cases of Morpho), leaving the field of the cell plain and 

 without a speck. The suggestion is strong that the whole series of 

 rings may have been formed by some one central disturbance, somewhat 

 as a series of concentric waves may be formed by the splash of a stone 

 thrown into a pool. It is especially interesting to remember that the 

 formation even of a number of concentric rings of different colours 

 from an animal pigment by the even diffusion of one reagent from 

 a centre occurs actually in Gmelin's test for bile-pigments. Bile is 

 spread on a white plate and a drop of nitric acid yellow with nitrous 

 acid is dropped on it. As the acid diffuses itself distinct rings of 

 yellow, red, violet, blue and green are formed concentrically round it 

 by the progressive oxidation of the bile-pigment. 



If the experiment is made by letting a drop of the acid fall on 

 a piece of blotting-paper wetted with bile, a fairly permanent imitation 

 of an ocellar mark can be made. It will be noticed that as in the 

 natural eye-spot, so here, the outermost zone appears first and the central 

 colour last. As also is usually the case in the ocellus, when all the 

 zones are formed, the centre may greatly increase in diameter without 

 any increase in the breadths of the circular zones, which merely get 

 larger in diameter, remaining of the same breadth. 



There is of course no reason whatever for supposing that ocelli are 

 actually formed by the oxidation or other simple chemical change of 

 the pigments of the field, but this example is merely given as an 

 illustration of the possibility that a series of discontinuous chemical 

 effects may be produced in concentric zones by a single central disturb- 

 ance. Indeed, that the formation of an ocellus cannot be in reality of 

 such simplicity is shewn by the fact that the scales of the centres of 

 ocelli generally exhibit interference-colours (usually white or blue) and 

 are then wholly or partially without pigment, while in not a few cases 

 the centres of ocelli are deficient in, or destitute of, scales. It must 

 also be remembered that occasionally the colour of one of the outer 

 zones is repeated in an inner zone, which would scarcely be expected on 

 the analogy of the oxidation of bile-pigments. 



(3) As in the case of Teeth at the ends of series, disappearance 

 of a member of a close series of eye-spots commonly occurs by the 



