108 EVOLUTION OF COLOR PATTERN IN LITHOCOLLETIS. 



thrown into a fold. In the overwintering generation^ the larvse hibernate, 

 changing to pupse in spring without spinning cocoons. The characteristic which 

 distinguishes the imagoes of this group from the usual type is that the dark 

 scales appear on the outer edges of the white streaks and fasciae; if a margin is 

 present on an inner edge, it is much less pronounced. 



The wide geographical range and the large number of species would lead to 

 the inference that the genus is an old one. If so, we would expect to find some 

 species which have halted at an early stage in their evolution and have therefore 

 preserved a color pattern which should bear some resemblance to the primitive 

 one and hence afford a clue to it. Others would have advanced farther, develop- 

 ment ceasing at different levels, so that the numerous differentiations produced 

 should constitute a chain of related species, broken no doubt by numerous gaps 

 but still sufficiently complete to indicate the different directions which evolution 

 has taken. The problem of this research is, then, to determine what is the primi- 

 tive color pattern of this group and knowing it to trace the paths along which 

 evolution has proceeded in the production of the respective patterns of the 

 numerous species now in existence. It is scarcely probable that the primitive 

 pattern is preserved among any of the earlier genera from which Lithocolletis 

 is descended. Such a long period of time has elapsed since the origin of Litho- 

 colletis that the species now belonging to ancestral genera have probably deviated 

 widely from the type of marking then characteristic of them. This view is 

 rendered more plausible by the fact that the subgenus Cremastohombycia, which 

 possesses structural generic characters which suggest it as the immediate ancestor 

 of Lithocolletis, has a type of marking differing considerably from that common 

 to the large and widely distributed typical group and resembling, to a marked 

 degree, the type of marking characteristic of some of the species of the smaller 

 and seemingly younger group, usually known as the '^flat-larval group. '^ If 

 so, we should search for the primitive color pattern among the less differentiated 

 species of the typical group of Lithocolletis. 



If evolution takes place in several definite directions irrespective of utility, 

 as contended by Eimer, this group should offer an opportunity of testing this 

 theory, within certain limits, since many of the highly specialized differentia- 

 tions, indicating a high phylogenetic development and requiring a long time to 

 perfect, are of almost microscopic proportions and of no conceivable value in 

 the life of the organism. Hence it is improbable that natural selection would 

 have stepped in to mar any results which might have been produced through 

 orthogenesis. 



In a genus such as this, where the color combinations are relatively simple 

 and the limits of the marks clearly defined and the pigmental colors belong to 

 the single series of yellows (or reddish yellows), browns and blacks, the problem 

 of tracing their differentiation both ontogenetically and phylogenetically should 

 be one less complicated than in higher groups, and the observations should 

 offer more direct testimony as to the means by which such results are obtained. 



