354 THE BIOLOGY OF INSECTS 



latter being almost circular black areas, each enclosing 

 patches of blue or purple and surrounded by a broadish 

 yellow border, while the '' eye-spots " in the forewings are 

 less definite, the inner boundary of each being yellowish 

 while the outer is formed by a series of yellowish, blue, and 

 lilac spots or patches, which represent the set of white spots 

 that are conspicuous in allied species at the same region of 

 the forewing. The central area of the forewing eye-spot in 

 V. to is partly of the blood-red hue of the general wing-area 

 suffused with black scaling. From the typical form in 

 which this black scaling is dense, and the rounded outline 

 of the forewing eye-spot well marked, a series can be 

 traced through which the black scaling fades away and the 

 outer pale patches becoming feebler, the characteristic 

 forewing eye- spot is no longer distinguishable — the variety 

 fischeri of M. Standfuss (1896). 



It is important, however, in the study of variation to 

 remember that continuity cannot be inferred simply from 

 arranging a graded series among specimens collected hap- 

 hazard from various localities. A frequency of the various 

 forms in relation to the general population, so that the 

 extremes tail off from a " modal " form corresponding more 

 or less closely to the mean value of the character under con- 

 sideration is a condition necessary to establish the case as 

 one of continuous variation, and this is more readily tested 

 where some quantitative feature, such as the size of the 

 whole insect or the proportionate size of some part of its 

 body to the whole, is under observation rather than such 

 a feature as colour pattern. A well-known and very in- 

 structive investigation on these lines was made on a large 

 number of male earwigs all collected on the Fame Islands 

 off the coast of Northumberland and described by Bateson 

 (1894). The forceps of male earwigs are longer than those 

 of females and more strongly curved, their length varying 

 to such a degree that the existence of two or three different 

 species has been imagined from this character. In the 

 specimens examined the actual length of the forceps varied 

 from 2 mm. to 9 mm., and a series could be arranged 



