98 HISTORY OF THE HUMAN BODY 



a given area), in texture, or in form; it may be increased to a 

 thick, matted wool, or may show every degree of reduction 

 down to a total loss. It may develop into bristles, as in the 

 hog, or even form spines, as in the hedgehog and porcupine, 

 although this latter result is usually brought about by the con- 

 fluence of numerous individual hairs. The " horn " of the 

 rhinoceros is such a structure, and not a true horn. Many 

 variations in thickness are brought about by modifications in 

 the hair groups, or by the interpolation of supernumerary hairs 

 independent of the group system. In the former case the num- 

 ber of single hairs in each group may be increased, or each 

 primary hair may be represented by a bundle ; or again, each 

 primary hair may be accompanied by a series of accessory 

 hairs, arranged as satellites about the former. In the latter 

 case there is usually a marked difference between the hairs 

 that are included in the primary system and those that are not, 

 as is seen in the case of the hog, in which there are two sizes 

 of bristles, coarse ones in groups, and finer ones interpolated 

 without system (Fig. 20, c). 



It may be said in general that arctic forms and those liv- 

 ing at high altitudes are the most plentifully supplied with 

 hair, while tropical and sub-tropical forms are sparsely cov- 

 ered. An aquatic life tends to reduce the hair coat; if the 

 animal is but semi-aquatic, as seals and otters, the hair is 

 reduced to the form of a fine plush, but in the Cetacea and 

 Sirenia, which are wholly aquatic, the reduction is almost 

 a complete one. Many apes are but scantily supplied with 

 hair, the ventral side of body and limbs being but sparsely 

 covered, while the upper part of the face and ears are nearly 

 bare. The same tendency is continued farther in Man, who 

 shows considerable racial variation, ranging from the hairy 

 Airius and certain hairy individuals in the white race to the 

 smooth and beardless Malays. That Man was formerly sup- 

 plied with a thick coat of hair, however, is shown by the fetal 

 condition, at one stage of which the entire body, not excepting 

 the face, is covered by a coat of fine down, the lanugo. This 

 mainly disappears before birth, and becomes eventually re- 



