EVOLUTION OF COLOR PATTERN IN LITHOCOLLETIS. 115 



across the wing thus separating a basal patch of dark scales from the band of 

 similar scales beyond it. In this species^ therefore^ the number of distinct 

 patches or bands distinguishable along the costa is seven^ separated more or 

 less completely from one another by white interspaces. 



The remainder of the species^ and by far the most numerous^ have been 

 included in one large group^ because all can be traced to a common origin and 

 all are manifestations of a similar tendency in development. All are charac- 

 terized by the presence of pairs of opposite curved, oblique costal and dorsal 

 streaks, widest on the margins and tapering in the middle of the wing to pointed 

 apices. There are a number of well-defined subgroups, due to the halting of 

 individuals at different stages in the general process of evolution and the differ- 

 entiation of a number of species from each of these. 



Standing at the foot of the group and somewhat apart from the other 

 species, is auronitens (Fig. 10). In it, there are three dorsal streaks before the 

 tornus,^ but little oblique and not displaced proximally from their normal position 

 with respect to the nervures. 



Argentinotella (Fig. 11), occitanica and leucothorax (Fig. 13) are also 

 characterized by the presence of three streaks on the dorsum before the 

 tornus. These are decidedly oblique, especially in leucothorax, in which there 

 is also a well-defined apical spot. In occitanica, the first, fourth and fifth costal 

 streaks present in argentinotella are wanting. 



In fitchella (Fig. 14), there are but two dorsal streaks before the tornus; 

 the first is unusually large, suggesting from its position the possibihty of its 

 formation from two white streaks. That this has occurred and the processes 

 bringing it about wih be discussed later. 



Salicifoliella (Fig. 15) and tremuloidiella (Fig. 16) are not closely related to 

 any of the large subgroup of species soon to be discussed. These two isolated 

 species are the result of differentiation and advancement along other lines, when 

 there has been a cessation, at a lower level, of the common course of evolution, 

 which if continued, culminates in such forms as hageni (Fig. 17) and later, in 

 the large subgroup of species of which cratcegella (Fig. 36) and obscuricostella 

 (Fig. 25) are typical representatives. Because of this, their markings will be 

 better understood when the laws governing their transformation have become 

 apparent. 



Insignis (Fig. 20), hageni (Fig. 17) and arbutusella (Fig. 18) are properly 

 grouped together. In another variety of insignis, the white extends from the 

 basal streak across the wing to the dorsum, forming a quadrate white patch similar 

 to that of hageni. These three species are likewise differentiations from an offshoot 

 of the main stem, although at a higher level than the two preceding species. 



We now come to the large subgroup of species characterized by two oblique 



1 The three margins of the wing are the costa or upper margin, the termen or outer margin 

 and the dorsum or inner margin. The junction of the costa and termen is the apex of the wing; 

 the junction of the termen and dorsum is the tornus. In this group, where there is no defined 

 termen, the tornus is the place where the hue of scales in the ciha meets the dorsal margin. 



