8<S THE CANADIAN ENTOMOLOGIST. 



evolution, by displaying a progressive series of minute gradations of 

 pattern ; a fourth plate might be devoted to an accessory suite of 

 specimens illustrating subordinate lines of variation; and on the final 

 plate an interesting series of aberrant examples could be presented in an 

 order suggestive of their systematic relationship. Five plates, so 

 managed, would furnish a pictorial analysis of tlie pattern-building 

 method of Petrosa more instructive than a laborious essay. 



Limited to twenty figures, I found it advisable to select a set of 

 examples suitable for a merely synthetic plate, indicating the leading 

 results of the pattern development rather than its progressive details. 

 The plate contains figures of eleven Petrosa males, and seven females. 

 A somewhat fuller series of variations could have been shown by figuring 

 only males. By selecting partly females, however, a distinct advantage 

 was gained, as I have included four appropriate examples bred from one 

 lot of eggs, and incidentally a direct proof is thereby supplied that the 

 plate represents in its local specimens a single valid species. Numbers i 

 to 9, inclusive, also ii and i6, are males. Numbers 12, 13, 14, and 17 

 to 20, inclusive, are females. For comparison, two instances of Nemeo- 

 phila plantaginis are included, numbers 10 and 15. The former is a 

 male, form Hospita, from northern Finland ; the latter a female, from the 

 Amour region in eastern Siberia. 



My analysis oi Petrosa is based on examination of 199 males and 160 

 females. The series is as complete as can be desired, comprising all 

 specialties and stages of pattern caught or bred during seven collecting 

 seasons. The entire material was first assorted in an order expressing 

 the gradual modification of pattern, beginning with initial " Scutidcri," 

 and progressing to ultimate " Petrosa.'' In order to estimate in numer- 

 ical terms the prevailing tendency or present attitude of the moth as to 

 pattern, the extended column of variation has been sectioned into 

 separate masses, thus distributing into convenient sections the pattern- 

 distance between the two extremes of variation. 



One hundred and seven flown males are first considered. Section i 

 contains 14 of these specimens, which, as to upper surface at least, are 

 formal Scudderi. Fig. i of the plate differs from the specimens of section 

 I merely in having the light coloured spur (which extends from base of 

 f. w. and is a rudiment of the longitudinal stripe of Petrosa) extended to 

 greater length ; in other respects it is Scudderi. Fig. 7 has this basal 

 spur as in Scudderi. 



