CHAPTER XII. 



linear series — continued. COLOUR-MARKINGS. 



Ocellar Markings 1 , especially those of Lepidoptera. 



Upon the bodies of animals belonging to many classes are 

 markings which consist of a central patch of colour surrounded by 

 a variable number of concentric rings of different colours. Such 

 markings are known as ocelli or eye-spots from their resemblance 

 to the pupil and iris of vertebrates. Eye-spots are perhaps best 

 known in Lepidoptera, but similar markings are not unfrequent in 

 other groups and especially on the feathers of Birds and in Fishes. 



In one of the best known chapters in the Descent of Man 2 the 

 nature and mode of evolution of these markings is the subject of 

 a full discussion, the case of eye-spots on feathers being chiefly 

 taken in illustration. As is well known, Darwin by the compara- 

 tive method, comparing the eye-spots found in different species, on 

 the different feathers of the same bird, or on different parts of the 

 same feather, found that it was possible to construct a complete 

 progression from a plain spot to a fully-formed ocellus. Though 

 no one examining such a series can possibly doubt that the simple 

 spot and the fully-formed ocellus are really of the same nature and 

 that the one represents a modification of the other, there remains 

 nevertheless the difficulty that members of a series of parts cannot 

 be assumed to represent conditions through which the other mem- 

 bers of the same series have passed, and it is of course clear that 

 the conditions found in some forms do not necessarily correspond 

 with phylogenetic phases of other forms. In the present instance 

 however Darwin is not specially urging this view, but brings 

 forward the comparative evidence chiefly in illustration of the 

 possibility that such structures may exist in an imperfect state 

 and so may be conceived of as having had a gradual origin. 



1 The evidence concerning eyespots of Lepidoptera is taken here because eyespots 

 when repeated in series, though borne on appendicular parts, are nevertheless 

 arranged chiefly with reference to the chief axis of symmetry of the body. In some 

 few forms, e.g. Taygetis, there is a conspicuous Minor Symmetry within the limits 

 of a single wing (the posterior), but this is not often the case. 



2 Descent of Man, 1871, n. pp. 132—153. 



