CHAP, xii.] EYE-SPOTS. 289 



Though doubtless the eye-spots of Birds are in their nature 

 not different from those of Lepidoptera yet their manifestations in 

 the latter are usually in some respects simpler than they are in 

 Birds. From the abundance of material also the Variation of eye- 

 spots is most easily studied in Lepidoptera and it is to them that 

 the present evidence chiefly relates. 



In preface to the evidence a few remarks are needed to direct 

 attention to certain features in the mode of normal occurrence of 

 eye-spots and in the manner of their Variation. 



On a survey of the facts it is at once seen that eye-spots are 

 extraordinarily variable both in number and size, some of the best 

 formed being occasionally absent, and large and perfect ocelli being 

 sometimes added in situations having normally no trace of such 

 marks. With this fact Darwin was well acquainted and he refers 

 to observations in illustration of it. In speaking of Gyllo leda he 

 concludes that from the great variability of the eye-spots " in cases 

 like these, the development of a perfect ocellus does not require a 

 long course of variation and selection ; " and again, that bearing in 

 mind " the extraordinary variability of the ocelli in many Lepi- 

 doptera, the formation of these beautiful ornaments can hardly be 

 a highly complex process, and probably depends on some slight and 

 graduated change in the nature of the tissues." The facts to be 

 given and the circumstances attendant on the variation of ocelli 

 tend to support this conclusion. 



Considered from the point of view of Meristic Variation the 

 chief feature in the manner of occurrence of eye-spots in Lepi- 

 doptera is the frequency with which they are repeated. A single 

 spot may be repeated in homologous places in both pairs 

 of wings ; in other cases there is a series along the margins 

 of one or both wings. Besides the repetitions thus occurring it is 

 especially worthy of notice that ocelli are very commonly repeated 

 on both surfaces of the wing (Satyridas, &c.), the centres of the 

 upper and lower ocelli coinciding. It need scarcely be remarked 

 that this effect is not produced by transparency of the wing-mem- 

 branes and scales, but is an actual repetition, the scales of both 

 surfaces being so coloured as to form an eye-spot on each side, the 

 two having their centres coincident. In some cases, e.g. Saturnia 

 carpini (the Emperor Moth), the rings and centres of the upper 

 and lower ocelli have nearly the same colouring, but in the majority 

 e.g. Pararge megcera (The Wall), Erebia blandina, &c., the upper 

 and lower spots, though coincident, have quite different colours. 

 In considering the Variation of the spots these facts as to the 

 repetition of the spots should be remembered, for, as has been often 

 insisted on in other cases of repetitions, we are concerned with the 

 evolution of the series and not of one member only. Here there- 

 fore regard must be had to the degree of correspondence between 

 the variations of the eye-spots in the fore and hind wings, on the 

 B. 19 



