Chap, ix.] HAIRS OF MAMMALS. 369 



account of their arrangement in different forms. 

 Hair is almost entirely absent from the body of adult 

 Cetacea, and only scantily developed in the Sirenia ; 

 this common character must not, however, be regarded 

 as any evidence of community of origin or closeness 

 of relationship, but rather as the result of exposure 

 to similar conditions. Sometimes, when the hair is 



Fig. 157. The Armadillo. 



scanty on the body, as in the rhinoceros, a number of 

 hair-like shafts unite to form a horn. In forms which 

 live in very cold climates, like the musk-sheep (Ovibos 

 moschatus), the hair is exceedingly long and thick, and 

 serves as an efficient protection against the external 

 cold ; the most striking example of this is afforded by 

 the thick coating of the extinct mammoth, which 

 lived in cold regions, whereas its allies, the elephants, 

 which, in recent times, are confined to warm coun- 

 tries, have but little hair. The soft hair may be 

 replaced by firm and strong spines, as in the porcu- 

 pine or hedgehog, where, thanks to their power of 

 Y 16 



