EVOLUTION OF COLOR PATTERN IN LITHOCOLLETIS. 157 



when formed is therefore nearer the dorsal margin. The chief forces acting in 

 the evolution of the pattern in this subgenus are the more rapid growth distad 

 of Band II as compared with Band III and the extension proximad of the ex- 

 tremities of Bands III and IV. The sequence of the species is as follows: 

 grindeliella (Figs. 59a, 5%), solidaginis (Fig. 60), ambrosiella (Fig. 61), 

 ignota (Fig. 62), verbesinella (Fig. 63). Grindeliella shows the basal streak in 

 actual process of formation; where the dorsal arm of the first fascia is wanting, 

 the black scales at the tip of the basal streak are homologous with those 

 on the outer margin of the fascia. All show the tendency which arises inde- 

 pendently throughout the ^^flat-larval group'' also, toward the complete fusion 

 of Bands V and VI. None of these species can be regarded as very primitive. 

 The similarity to a group of species in the division of the genus shortly to be 

 discussed is due to the fact that the same laws underlie their evolution. 



A tendency similar to that witnessed among several of the species of the 

 first division of the genus, toward the preservation of Bands II and III in a shape 

 approaching the primitive, prevails among many of the species of the second 

 division of the genus. Obstrictella (Fig. 64) has preserved Bands II, III 

 and IV in a very primitive condition; during its differentiation, however, the 

 white streak at the base has been entirely obliterated. 



Above this level, evolution takes place in four divergent directions, as repre- 

 sented on Plate IV. 



In one of these branches. Bands II, III and IV tend to remain separate. 

 Within this group, the first step in evolution is the outward extension of the 

 middle of Band IV to meet Band V; at the same time the bands become displaced 

 very slightly toward the base of the dorsum; the fasciae between them are not 

 as yet angulated. Specific differentiation at this level has produced tubiferella 

 (Fig. 65), in which all white markings beyond the fasciae, except sometimes 

 a minute white dot before the apex, have been suffused with ground color; 

 aceriella (Fig. 66) and hamameliella (Fig. 67), the latter more advanced 

 because of its deeper ground color; ostryarella (Fig. 68), corylisella (Fig. 69), 

 cesculisella (Fig. 70) and guttifinitella (Fig. 71). The loss of the white 

 streak at the base seems to be accomplished after the principal changes 

 in the configuration of the ground color have been brought about, that is, 

 after specific differentiation has set in. The fact that the white streak 

 extends entirely across the wing, separating Bands I and II completely in 

 such species as hamadryadella (Fig. 77) and umbellularice (Fig. 78) which are 

 by no means primitive in regard to the shape of the areas of ground color, and 

 observations on the development of color in the pupal wings of cesculisella, 

 support this hypothesis. 



In carycefoliella (Fig. 72) and lentella (Fig. 73), the bands have become 

 quite distinctly though obtusely angulated. 



Above this level evolution has taken divergent paths. In one direction, 

 evolution in the shape of Bands II, III and IV has progressed in the same manner 



