22 NELSON 



number of species and subspecies of these squirrels, together 

 with their great individual variation, have hitherto proved an 

 insuperable stumbling block in their treatment. The large 

 series now available for study shows that the group as a whole 

 is in a state of evolution and has developed groups of closely- 

 related species or well marked geographic races, often within 

 very limited areas. For example, in the State of Vera Cruz, 

 the typical form of Sciurus aureogaster occurs throughout the 

 northern part, ^5'. aureogaster frumentor on the west-central 

 border, and S. aureogaster hyfopyrrhus in the southern part. 

 Many of the subspecies are so different from others of the same 

 species that without the intermediate series no one would for a 

 moment suspect their specific identity. 



The extraordinary amount of geographical variation in trop- 

 ical North American squirrels is due mainly to an unusual plas- 

 ticity of organization which allows slight climatic differences to 

 produce a visible effect. The most obvious of these influences 

 are differences in temperature and rainfall with their distribu- 

 tion through the year, and consequent effect on the vegetation. 



The cool forests of the Transition zone in the interior of 

 southern Mexico and Guatemala are characterized by moderate 

 rainfall and have what may be called an inland climate. The 

 forests of the same zone on the mountains bordering the hot coast 

 plains, exposed to the moisture bearing winds from the sea, re- 

 ceive a much greater annual rainfall accompanied by cloudiness 

 and mists, resulting in an exuberance of vegetation not found 

 elsewhere in this zone. The effects of these differences in hu- 

 midity upon the squirrels are beautifully illustrated by Sciurus 

 ■polio^us and its subspecies. For example, subspecies hernan- 

 dezi^ nemoralis and cervicalis, of the drier interior mountains, 

 are characterized by dull grayish upperparts and white under- 

 parts, and the absence of sharply contrasting nape and rump 

 patches. Sciurus -polio^us and its subspecies effugius inhabit 

 mountains near the coast, and the effect of the increased humidr 

 ity is marked by the striking contrast between their gray backs 

 and bright ferruginous underparts. In addition, effugius has 

 an unusually dark nape patch and brilliantly white ear patches. 



Increased humidity within the tropics, also, is usually accom- 



