96 THE AMERICAN NATURALIST: [VoL. XXXVI. 
evidently represent an extreme adaptation to the open, dry and 
sunny, and more or less grass-covered regions of the New 
World. Such regions are, perhaps, most typically represented 
by the deserts of Wyoming, the plateau of central and northern 
Mexico, and the pampas of La Plata. The area occupied by 
the genus and extending, as above stated, from Montana to 
Argentina, presents in North America an eastern offshoot to 
Florida (P. badius) 
and a peculiar west- 
ern offshoot to the 
Sandwich Islands 
(P. occidentalis). In 
South America there 
is between Colombia 
and Argentina a con- 
siderable area from 
which species of 
Pogonomyrmex are unknown, but the continuity of the distri- 
bution, though broken at this point, is at least in part preserved 
further to the east by the Brazilian P. negelii. 
This distribution over two continents naturally suggests an 
inquiry as to whether the species arose in North America and 
migrated thence along the Andes into South America, or had 
their origin in the pampas of Argentina and migrated into 
North America over the same lofty road. Two authors, 
v. Ihering (94, p. 416) and Emery (94, p. 354), who have seri- 
ously studied the interesting problems suggested by the dis- 
tribution of the American ants, agree in regarding North 
America as the primeval home of the species of Pogonomyr- 
mex. Concerning this genus and the genera Dorymyrmex 
and Forelius, which have a very similar distribution, Emery 
says: 

Fic. 7. — Pogonomyrmex californicus Buckley. Worker. 
Their migration probably proceeded along the Andes at a time when 
the climate was cooler and the vegetation therefore different from the 
present. Later, on the supervention of new floral conditions, they were 
crowded out of a portion of their former domain by the tropical ant fauna. 
For the reason that the southern species of Pogonomyrmex and Dorymyr- 
mex are more numerous than the northern, we might, perhaps, assume that 
these animals had migrated from the south to the north. But it is not in 

