GENERATION OF ANIMALS, V. iii. 



scanty ; beside this, it also shows diflPerences of 

 colour : it may be white or black or any shade be- 

 tween these two. Some of these differences are also 

 exhibited by the hair according to the various times 

 of life, youth and more advanced age. This is 

 noticeable chiefly in the case of human beings. Thus 

 the hair gets shaggier as age advances, and some 

 people go bald in front. Children do not go bald, 

 nor do women ; men do, however, when they begin 

 to get on in years. In human beings, the hair on 

 the head turns white as age approaches ; in other 

 animals, however, this does not noticeably occur : 

 the horse is the one which shows it most. Human 

 beings go bald on the front of the head, but they go 

 grey first on the temples ; none however goes bald 

 either here or at the back of the head. As for those 

 animals which have no hair but the counterpart of 

 hair instead (thus, birds have feathers, and the fish 

 tribe have scales) — in them some conditions of the 

 kind described occur in a corresponding way. 



We have already stated in the treatise on the 

 Causes of the Parts of Animals " the purpose for the 

 sake of which Nature has made hair in general and 

 provided animals with it. The business of our present 

 investigation is to show what are the pre-existing 

 circumstances, what are the factors of necessity, on 

 account of which the particular sorts of hair occur. 



The chief cause, then, of its thickness and thinness 

 is the skin ; which in some animals is thick, in others 

 thin ; looseknit in some, compact in others. A con- 



" P. A. 658 a 18; viz., for the sake of shelter and protec- 

 tion. 



s 513 



