ЖҮРЧҮ UE E D ОНГИН RTT NERA TR ERR ee تمو تف‎ Е 
ee a و‎ 
EVOLUTION ОЕ COLOR PATTERN IN LITHOCOLLETIS. 153 
If on the dorsal margin, the retraction of pigment from the outer edge of 
Band III takes place more rapidly than the extension proximally of the extremity 
of that band, its dorsal portion will disappear, and the second white dorsal 
streak will coalesce with the first; fitchella (Fig. 14) typifies this result. The 
extension of color onto a white streak has probably taken place most rapidly 
just within the dorsal margin; the evidence for this conclusion rests upon the 
observed concavity of the outer edges of the white streaks and their greater 
width on the margin. 
If the most rapid movement, as is usually the case, has been that of the outer 
edge of Band II toward the base, the first result will be the union of the upper 
inner edge of the first white dorsal streak with the short oblique streak at the 
base separating Bands I and II. A small streak of ground color will be left 
between them on the dorsum; this soon becomes a small spot removed from 
the dorsal margin by the more rapid shrinking away of Band II on the margin. 
Salicifoliella (Fig. 15) and tremuloidiella (Fig. 16) have been differentiated as 
species at this level. 
The small spot of ground color just referred to becomes smaller and smaller 
and finally disappears. The base of the dorsal margin is then occupied by a 
white patch whose outer edge is the inner edge of Band III. Differentiation at 
this stage has given rise to hageni (Fig. 17), arbutusella (Fig. 18) and insignis 
(one variety) (Fig. 19). In hageni and insignis, the first white costal streak 
has disappeared entirely. Following closely behind the retraction of pigment 
from the outer edge of Band II on the costa, the extremity of Band III has 
been produced along the costa to the base; phylogenetically then, this is the 
second time pigment has appeared along the base of the costa. In arbutusella 
(Fig. 17) the first costal streak has been involved in a change similar to 
that of the first dorsal with which it has united. In these three species equally 
distinet dark margins have appeared on either side of the white streaks, and 
there has been a very decided darkening of the scales of the ground color between 
the streaks, indicating that after the general phylogenetic evolution of the 
pattern of the ground color had come to a standstill, specific differentiation pro- 
ceeded rapidly and for a considerable period of time. In the other variety of 
insignis (Fig. 20), Band III in its dorsal half has been produced to the 
base, most rapidly just above the dorsum, leaving a white median basal streak 
and a white spot on the dorsum near the base. This then is the method by which 
a median long basal streak has originated; its basal part is therefore homologous 
with the white streak separating Bands I and II; its apical half is homologous 
with a part of the white streak separating the dorsal portions of Bands II and III. 
Where the extension of Band III to the base has followed immediately on 
the progressive withdrawal of pigment from the outer edge of Band II, without 
the intervening cessation in development found in insignis, the configuration of 
the eolor areas is that of the group of species of which obscuricostella (Fig. 25) 
and crategella (Fig. 36) are typical examples. In most of the species of this 
