CLIMATIC VARIETIES 181 



of plants are to the well-developed individuals of the lower 

 levels. I do not anticipate that factorial differences will be 

 found in these insects, and it is by no means impossible that the 

 distinctions between them are the direct consequences of altered 

 conditions. The relations of arcania to satyrion are more doubt- 

 ful, and in that case a factorial difference may at least be sus- 

 pected. 



The species of the genus Setina have Alpine forms which 

 agree in possessing a characteristic extension of the black pigment 

 to form radiating junctions between the spots on the wings. 

 Speyer, who discussed the interrelations of these forms in detail, 22 

 lays stress on the absence of genuine transitional forms between 

 aurita and the variety ramosa. Both are mountain insects but 

 ramosa extends to levels higher than that at which aurita ceases, 

 which is about 4,000 feet. The two forms are often found flying 

 together. Speyer says that his brother searched diligently for 

 transitional forms at the level of overlapping, but found none, 

 so that at least they may be regarded as rare. The variety 

 ramosa is not infrequent at much lower levels (e. g., Chiavenna, 

 1,020 feet; Reussthal, 1,500 feet) and extends as high as the 

 permanent snows. In the British Museum collection, however, 

 I have seen several that I should regard as transitional. Speyer 

 perhaps would have classed as ramosa all in which the spots of 

 the central field were united, and it is by no means unlikely that 

 breeding would prove such individuals to be heterozygous. 23 



22 Speyer, Stettiner, Ent. Ztg., XXXI, 1870, p. 63. 



23 In regard to the closely analogous case of Spilosoma lubricipeda, Standfuss 

 makes a similar statement. He bred the type on a large scale with the radiate form 

 which he calls intermedia, and says that in four years of miscellaneous crossing he 

 never obtained really transitional forms. Nevertheless after examining large series, 

 especially those of Mr. W. H. B. Fletcher, I came to the conclusion that several 

 might be so classed, but I am quite prepared to find that such specimens are hetero- 

 zygous. (See Standfuss, Handb. d. Gross- Schmet., 1896, p. 307.) It is by no means 

 unlikely that various dark forms of lubricipeda correspond with a progressive series 

 of factorial additions. Many of the stages have been named, and of these the most 

 definite are the intermedia of Standfuss (probably = eboraci of Tugwell) and the 

 very dark Zatima of Heligoland, in which only the thorax, the nervures and a small 

 field in the forewings remain yellow. A form was bred by Deschange from Zatima 

 in which even the field in the forewing is obliterated. The exact circumstances in 

 which Zatima occurs in Heligoland would be worthy of special investigation, for the 



