150 EVOLUTION OF COLOR PATTERN IN LITHOCOLLETIS. 
species of the first division are more closely related among themselves than to 
any of the species of the second division or of Cremastobombycia. 
An ancestral form, more primitive in respect to generie structure as well as 
color pattern and transversely marked with seven bands of a uniform pale 
yellowish color, has evidently given rise first to a series of species in which dark 
margins have developed earliest on the outer edges of bands—in this case, the 
white streaks and fascie are said to be internally dark margined. Later, or 
perhaps contemporaneously, in another species of the same ancestral strain, 
the opposite tendency has developed, and there has been evolved a series of 
species in which dark margins appear earliest on the inner edges of the bands— 
in this ease, the white streaks and fasciz are said to be externally dark margined. 
A few of these latter species, constituting the subgenus Cremastobombycia, have 
retained a more primitive generic structure; the remainder have followed the 
same course of evolution structurally as the species of the first division of the 
genus. It is, of course, possible that this second tendency originated twice 
and independently. 
Several factors combine to render it probable that Cremastobombycia (in 
its modern form) and the “flat-larval group” originated at a somewhat later 
period than the typical Lithocolletis (Fig. 26). The restricted geographical 
range of the “‘flat-larval group" and of Cremastobombycia, which with one 
exception are confined to America, indicates their comparatively recent origin. 
Considerations based upon larval characters confirm this view. The later larval 
stages of the typical Lithocolletis are identical with those of Gracilaria, which is 
without question accepted as the ancestor of all these groups. The corresponding 
stages in the larva of Cremastobombycia have a more flattened and triangular- 
shaped head, without, however, the structural modifications which have de- 
veloped in the “‘flat-larval group." It may be concluded, then, with reasonable 
certainty, that after the origin of the typical Lithocolletis, the species of the 
ancestral stock retained for a short period their imaginal structure. During 
this time, changes in the larval form were initiated. In the ancestral form of 
Cremastobombycia, development along this line did not progress far, and the 
larve have preserved the normal mode of feeding; hence there has been no 
attendant structural modification. 
Cremastobombycia may have originated after a certain amount of evolution 
in the color pattern had already taken place; this view is supported by the 
observation that none of its species conserves as primitive a type of marking as 
is to be found in several species of the “ flat-larval group." 
We may conclude, therefore, that the immediate ancestor of all the groups 
under discussion was of a somewhat more primitive structure than Cremasto- 
bombycia as we now know it. The structural relations of the four groups with 
reference to the ancestral stem are illustrated diagrammatically by Fig. 25; the 
relative time of origin of each by Fig. 26. "The similarity of the color pattern of 
the species of Cremastobombycia to that of some of the species of the second 
