GEOGRAPHIC DISTRIBUTION OF ANIMALS: CONTINENTS 



275 



FIG. 12.6. Cline exhibited in the striping of the legs of the common zebra {Equus bur- 

 chellii or quagga) in the different portions of its range in Africa south of the Sahara 

 Desert. (Striping patterns redrawn from Cabrera, "Subspecific and individual variation 

 in the Burchell zebras," Journal of Mammalogy, Vol. 17, 1936, pp. 89-112.) 



would be of no value to the organism in aiding its adaptation to the en- 

 vironment. 



Frequently, however, clines do involve adaptive changes; this fact is re- 

 flected in what have come to be called the zoogeographic rules. Berg- 

 mann's rule states that in warm-blooded animals the body size increases 

 with decrease in average temperature. This means in the northern hemi- 



