276 INTRODUCTION TO EVOLUTION 



sphere that body size increases in the northern parts of the range, as com- 

 pared to the size exhibited in the southern portions. Thus our common 

 deer averages larger in the northern parts of its range than it does in the 

 southern. The same is true of many other mammals and birds. Large size 

 in a cold climate has adaptive value in preventing loss of body heat. 

 Since the mass of a body (e.g., a sphere) increases as the cube of the 

 diameter while the surface area increases only as the square, larger bodies 

 have relatively less area through which to lose heat than do smaller ones. 

 Hence it is of value to a warm-blooded animal in a cold climate to be 

 large. 



Loss of heat through exposed areas of limbs, tail, and ears would also 

 be disadvantageous to an animal in a cold climate. Allen's rule states that 

 such exposed portions of the body decrease in size with decrease of aver- 



vl 



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V 



>\-i'-,.'„„ 



'■'T'V \ 



/ 



I 



^- I 



a b c 



FIG. 12.7. Head of arctic fox (a), red fox (b), and desert fox (c). (From Guyer, Animal 

 Biology, Harper & Brothers, 1948, p. 175; redrawn from Hesse, Allee, and Schmidt, 

 Ecological Animal Geography, John Wiley & Sons, Inc., 1937.) 



age temperature. Comparison of the size of ears of an arctic fox, a fox of 

 the temperate woodlands, and a desert fox illustrates the principle (Fig. 

 12.7). Of course the large ears of the desert fox may also have some posi- 

 tive adaptive value for that species. 



Gloger's rule states that "among warm-blooded animals those living in 

 warmer and moister climates develop more melanin pigment [are darker 

 than are animals in cold, dry climates], whereas forms in dry, hot climates 

 have more yellow and red pigment" (Goldschmidt, 1940). There are other 

 trends sometimes stated as "rules," — e.g., within bird species the num- 

 ber of eggs in a clutch increases from south to north. All of these rules 

 are generalizations, and all have their exceptions. They describe tend- 

 encies exhibited in general by many clines. 



The existence of clines themselves is perhaps the most interesting fact 



