1970] 
Carde y Shapiro , Clench — Lethe 
97 
— particularly the early stages — becomes better known. Similar 
cases recently uncovered in the Lepidoptera include the tortricid 
moths A r chips argyrospilus and A. mortuanus , which differ only in 
sex attractant and in some characters of the last-instar larva (Roelofs 
and Comeau, 1969), and the papilionid butterflies Papilio zelicaon 
and P. gothica, said to differ consistently only in host-plant specificity 
but to behave as species in genetic tests (Remington, 1968). The 
Holomelina aurantiaca complex (Arctiidae), often thought to consist 
of two species, actually includes at least ten, exceedingly similar in 
genitalic morphology, color and pattern, but differing in chromo- 
some number (Carde, unpublished). 
The seemingly inevitable problem with sympatric sibling pairs 
such as Lethe eurydice and appalachia is to account evolutionarily 
for the “elegant” manner in which they coexist. The view that 
reproductive isolating mechanisms and ecological differences evolve 
in response to deleterious hybridization and competition in secondary 
sympatry (Brown and Wilson, 1956) is now very widely accepted. 
It was recently challenged by Ehrlich and Raven (1969), who 
proposed that isolating mechanisms usually develop during the 
genetic differentiation of allopatric populations under different 
selective regimes. This is in effect a reformulation of the view of 
most nineteenth- and early twentienth-century evolutionists. At- 
tempting to explain the ecological relationship of a given set of sibling 
species requires consideration of the following points: 
1. The apparent absence of ecological interaction (e.g., competition) 
or gene flow between presently sympatric populations does not rule 
out such events in the past, nor for competition, in the future. 
Furthermore, intermittent large-scale gene flow between normally 
allopatric populations, associated with fluctuations in population sizes, 
has probably been an important component of speciation (Brown, 
1 95 7 ) * Such fluctuations could also result in episodes of competition 
between otherwise non-competing species. 
2. Biogeographical evidence may offer important clues to episodes 
of prior sympatry or allopatry in the evolution of species differences 
( cf . Mengel, 1964). 
3. In the absence of evidence for character displacement, it cannot 
be assumed that biological differences which appear to prevent com- 
petition evolved in response to the adverse effects of competition. 
The ecological differences among the American species of Lethe 
can be resolved into two parts: that involving the eurydice group 
alone and that concerning the eurydice and portlandia groups. The 
