340 
Psyche 
[December 
specific causes for the widespread failure of the 1970 adult brood 
are still uncertain and probably were cumulative. In a simpler case, 
Ehrlich et al. (1972) have noted a similar series of extinctions of 
the butterfly Glaucopsyche lygdamus at these altitudes in this same 
general area which they prove was due to destruction of oviposition 
sites and larval food plants by the snowfall of 26 June 1969. 
Consequences of the failure of the 1970 A. conspersa brood are 
the problem of subsequent recruitment and the entire question of the 
maintenance of the Black Mesa hybrid zone. It will be very in- 
structive to follow the reestablishment of the Arphia demes from 
the few surviving centers of the 1970 brood, from the off -mesa 
populations, and perhaps from the odd-year brood. Certain propor- 
tion discrepancies, such as those shown by #53b (the Forks), #J 2 b 
(Corral Gulch), and #45d(c) (southeast meadow of the Burn 
Area) seem related to alternate years. Since we have proved in the 
present results that the Black Mesa populations overwinter as egg 
and nymphs, there may be two geneticalty independent populations, 
one reaching adulthood on even-numbered years and one on odd 
years. However, further yearly censusing of strategic populations is 
necessary to determine the validity of this assumption and to deter- 
mine the amount of temporal crossing-over by any non-diapausing 
individuals. 
The narrowness of the Black Mesa hybrid-zone has puzzled us, 
but now it seems clear that periodic extermination of the mesa-top 
populations could afford a considerable setback to any extensive gene 
flow. Indeed, the presently inhabited portion of the mesa probably 
has been colonized by A. conspersa only within the past 150 years 
since the beginning of the last climatic warming trend after the 
Neoglaciation (Remington, 1968, p. 350; and Richmond, 1965) 
which probably had made most of the mesa-top uninhabitable. Any 
long term cooling trend resulting in increased precipitation, numbers 
of late frosts or snows, etc., could again cause the mesa to become 
an altitudinal barrier. It is also probable that the 1970 population 
crash is not a unique occurrence on the mesa, even during this present 
hypsothermal period. 
If the present zoogeographic pattern of wing-color variation is the 
result of recent colonization, it is probably safe to assume that the 
connecting corridors and forest barriers are at least as old and we can 
describe original invasion patterns in terms of the present topography. 
However, with current pressures on the National Forest Service to 
release tracts for lumbering, the original proportions of vegetational 
associations, the climatic patterns and even the water-holding capacity 
