144 EVOLUTION OF COLOR PATTERN TN LITHOCOLLETIS. 



black, without passing through the intermediate yellowish and brownish shades. 

 The tendency for dark pigment to develop along the edges of bands adjacent to 

 unpigmented areas is exemplified in this group also, with this difference, however, 

 that it appears first along the inner edge of a band. When dark pigment develops 

 in addition along the outer edge, it appears at a later period. The converse is 

 true in the first division of the genus; the dark margin appears earlier on the 

 outer side of a band. I can offer no explanation of the original cause of the 

 difference in direction of development; the result in either case seems to be the 

 expression of the same general tendency in development, acting, however, with 

 different degrees of intensity on either side of a band. It would seem that the 

 development of unusually dark pigment on one side of a band — the acceleration 

 of chemical processes over a localized area — retards development on the other 

 side. 



The ontogeny within this division affords very little evidence of the operation 

 of the first two processes of evolution defined earlier in this discussion. However, 

 the shape of the bands and the separation of the white fasciae into opposite spots, 

 with the total obliteration of some of the spots, is sufficient proof that these forces 

 have acted during the evolution of the species. 



L. hamadryadella (Fig. 77, PL IV) alone, of those observed, shows the out- 

 ward prolongation of the middle of a band; the gradual extension distally of 

 the dark dusting between the dorsal and costal portions of Band V + VI and 

 its final contact with the apical dusting (Figs. 22, 23) is an example of the 

 working of this law. The third law postulated to explain the displacement of 

 oblique streaks toward the base receives decisive confirmation from the adult 

 markings of the same species. A narrow line of scales crosses the wing beyond 

 the yellow portions of each of Bands II, III and IV, being separated from them 

 by white. These scales occupy positions corresponding to the outer margins of 

 these bands; ontogenetically, the dark pigment develops in them after it has 

 appeared on the inner margins of these bands. The conclusion is apparent that 

 these lines of scales mark the earlier phylogenetic limits of these bands. The 

 bands have shrunk away, most rapidly on the costa and dorsum, leaving behind 

 them the margins as visible evidences of the process. 



In L. tiliacella {Fig. 7), which, from its conservation of the greatest 

 number of bands in a primitive straight condition, the very pale ground color 

 and the entire absence of any dark scales in some of the bands, may be regarded 

 as the most primitive of the American species, there is a difference in the time 

 of appearance of the bands. In the more advanced species, the bands appear 

 at the same time. Evidently, then, in the higher species, there is a hastening 

 and crowding together of the earher stages, so that the bands which formerly 

 appeared in sequence now appear contemporaneously. Those in the apex of 

 the wing, which have experienced the least modification during the phylogenetic 

 development of the species, alone recapitulate the racial history during their 

 ontogenetic development. 



