EVOLUTION OF COLOR PATTERN IN LITHOCOLLETIS. 151 



division of the genus does not in any way indicate that this is primitive; it has 

 rather been produced independently in the two groups, by the action of the 

 same three fundamental factors. 



The phylogenetic tree thus shows two main stems extending in different 

 directions from a common origin. By the application of the three general 

 principles directing the evolution of the pattern of the ground color, it is possible 



seta 



fitkocclletis Tlat'larval 



fvrphurosela ^^ 





; S 





; ^ 



TUt -Ureal 



MkoJleU^ 1 



^roup. 



Jypical ^ 





'\ ^ 



/' 



. LreTnastohomhycia 



Cl-ncestral /orm, 



Fig. 25. Phylogenetic tree showing structural rela- 

 tionships of the groups of Lithocolletis . 



Clncestral Tcrrw. 



Fig. 26. Phylogenetic tree showing relative time of 

 origin of the groups of Lithocolletis. 



to retrace the steps in the origin of the different groups of species. The evo- 

 lution of the species of the ''cylindrical-larval group/' which will be discussed 

 first, is illustrated on Plate III. 



In all of the species except hataviella (Fig. 58), there has been a fusion 

 of Bands I and II along the costa. 



If evolution in the shape of Bands II and III has been in the same direction 

 and at the same rate, and the outward growth of the middle of each has not 

 been sufficient to bring it in contact with the band following, and this outward 

 growth has been compensated for by an equal growth toward the base of the 

 extremities of Bands III and IV, species characterized by two entire fasciae will 

 be produced, the first at the basal fourth, the second at the middle of the wing. 



Where evolution ceases early, all or most of the bands may be preserved as 

 transverse bands. L. tiliacella (Fig. 1) has been differentiated as a species 

 when, except for the fusion of I and II on the costa. Band IV alone had come 

 in contact with the band beyond. Specific differentiation, that is, the production 

 of dark margins, etc., has also halted soon, as we observe that no dark scales 

 are present on the last three bands. If development along the same line proceeds 

 farther, and by the farther outward growth of the middles of Bands II and III 

 and the extension inwardly of the extremities of III and IV, the fascise between 

 them become distinctly bent, the pattern of the ground color resembles that of 

 oregonensis. There has also been an accompanying gradual deepening of the 

 ground color. Specific differentiation at this level over a long period, as evi- 

 denced by the black apical spot, has given rise to oregonensis (Fig. 2). If, 

 on the other hand, evolution of the first three bands stops at the level of tiliacella 

 but continues in the apical four bands until IV, V, VI and VII are continuous 

 along the middle of the wing, species of the type of the European L. nicellii will 

 be produced. 



