1955] Creighton — Distribution of Novomessor 93 
It goes without saying that this response to elevation 
has a direct connection with the topography of the regions 
in which the two species occur. In subsequent pages the 
writer has discussed some of the topographic features 
which affect the distribution of the two species. The most 
extensive of these is the Sierra Madre Occidental, which 
determines the western limit of the range of both species 
for several hundred miles in northern Mexico. Most carto- 
graphers show the Sierra Madre Occidental as a continuous 
rampart which extends from Zacatecas northwestward to 
the southern border of Arizona. In the opinion of the 
writer it may be doubted that the highlands of Zacatecas 
ought to be considered as a part of the Sierra. At least 
they are cut off from the rest of the Sierra by the valley 
of the Rio Mezquital, which rises on the Plateau in south- 
ern Durango and flows westward to the Pacific. A similar 
gap, leading south to Aguascalientes, occurs at the eastern 
end of the highlands of Zacatecas. But proceeding north- 
ward from the valley of the Rio Mezquital to Latitude 
30° the main chain of the Sierra presents an unbroken 
barrier whose crest varies between 8000 and 9500 feet. 
It should be noted that there are few peaks in this region 
and none of them are high. Nevertheless the Sierra forms 
an effective barrier to any species whose vertical range 
is below the 8000 foot level. Both albisetosus and cocker- 
elli appear to be held to the eastern side of the Sierra 
until its topography changes in northwestern Chihuahua. 
North of Latitude 30° the Sierra breaks up into a number 
of scattered ranges, few of which show elevations in 
excess of 7500 feet. Between these ranges are broad 
valleys which communicate on the east with the Mexican 
Plateau and on the west with the narrow Sonoran coastal 
plain. From east to west the elevation of these valleys 
gradually descends from 5000 to 1000 feet. There is no 
barrier here comparable to the main chain of the Sierra 
further south and both albisetosus and cockerelli occur 
widely in this region. The range of cockerelli is, however, 
more extensive than that of albisetosus. The latter species 
does not descend below 2500 feet in this region, while 
cockerelli occurs at elevations down to 700 feet. It follows 
that cockerelli occasionally nests on the inner edge of the 
