MAMMALS OF NORTHERN COLOMBIA—HERSH KOVITZ 33 
A situation somewhat analogous to the above is presented by that 
of maracaibensis of the Maracaibo Basin and griseogena of the hilly 
northern coast of Venezuela. They are widely differing forms whose 
habitats are separated from each other by outlying ranges of the 
Cordillera de Mérida. The two races may intergrade along the 
coast, but no specimens are available from intermediate points to 
determine this. It has been shown, however, that both maracaibensis 
and griseogena grade into zuliae. 
The squirrels from the eastern and southern slopes of the Sierra 
Nevada de Santa Marta show clearly their origin from the lowland 
splendidus. Like the others, they become progressively smaller and 
darker, or agouti, with increasing altitude. Unlike the squirrels of 
the Cordillera Oriental, however, their underparts are consistently 
sharply defined white, as in splendidus. 
The squirrels of the highest forested altitudinal levels of the eastern 
Andes are of special interest. They are thickly furred, dusky, and 
more or less uniformly agouti. They: contrast markedly with the 
larger, brightly colored squirrels of the lowlands. So different is their 
appearance that they have been described as specifically distinct. It 
seems, however, that each population of these highland squirrels has 
been derived directly from the squirrels of the next lower altitudinal 
zone. In the absence of any barrier over a large, well-defined area of 
the highlands, the several heterogeneously derived populations may 
merge into a distinct and homogeneous group. This cbscures the 
fact that each population is the end product of a convergent, alti- 
tudinal cline which may have started, as in the case of variabilis, 
splendidus, and norosiensis, from three widely divergent forms in the 
lowlands. Examples of two populations of meridensis were examined. 
One series is from the Sierra de Mérida, the other from the Péramo de 
Tama. The former shows direct gradation into griseogena while the 
latter shows its derivation from specimens of zuliae farther down the 
same river system. Similarly, griseimembra, of the pdramos above 
Bogota, though hardly distinguishable from meridensis, represents a 
terminus of the altitudinal gradient of which chrysurus of Bogoté is 
next below. 
In the above it has been noted that at higher altitudinal levels the 
lowland. squirrels are represented by smaller forms. This tendency 
to reduction in size is continuous, frequently pari passu, to the highest 
altitudinal levels inhabited by the squirrels of the species. We have 
seen that the lowland forms are represented by darker ones at the 
next higher altitudinal zone. Where the lowland form is contrastingly 
colored, black and reddish, the next altitudinally higher one is less 
contrasted due to the augmentation or more even distribution of the 
black pigment of the body. This suppresses, to varying degrees, the 
otherwise contrastingly reddish sides and shoulder regions. Where 
