EVOLUTION OF 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 11. 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 tremidoidiella (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), arhutusella (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 arhutusella 

 (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 

 distinct 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 color areas is that of the group of species of which obscuricostella (Fig. 25) 

 and cratcegella (Fig. 36) are typical examples. In most of the species of this 



