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, 59b), 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 tendeney 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 faet 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 У; at the same time the bands become displaced 
very slightly toward the base of the dorsum; the fasciæ between them are not 
as yet angulated. Specific differentiation at this level has produced tubiferella 
(Fig. 65), in whieh all white markings beyond the fascie, 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 (F ig. 68), corylisella (Fig. 69), 
esculisella (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 umbellularie (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 esculisella, 
support this hypothesis. 
In caryefoliella (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 
