GENERATION OF ANIMALS, V. iii.-iv. 



shed their feathers or hair or leaves. In man, how- 

 ever, the spring, summer, autumn and winter of his 

 hfe are not seasons according to the calendar but 

 seasons of his own age ; so that, as these do not go 

 through the cycle of change, neither do the conditions 

 which depend on them ; although the cause which 

 controls the change of conditions is a similar one in 

 his case too. 



I think we have now discussed all the conditions 

 that affect hair, except that of colour. 



In the rest of the animals, the reason for the IV 

 various colours of the hair, and for its being single- ^ain^ **' 

 coloured or variegated, is the nature of the skin. In 

 man, however, this reason operates only in the case of 

 the greyness of the hair due to disease (as when the 

 hair becomes white during leprosy), not that due to 

 old age, and if the hair is white, the whiteness does 

 not derive from the skin. The reason is that the hair 

 grows out of the skin, and thus when the skin out of 

 which it grows is diseased and white the hair is itself 

 affected by disease, and disease of hair is grevness. 

 On the other hand, the greyness which is due to age Greyness. 

 is the result of weakness and deficiency of heat. 

 Every age of life tends to gra\'itate into chilliness 

 when the body's \igour declines, and especially when 

 this happens in old age, since old age is cold and dry." 

 We must bear in mind that the nourishment which 

 reaches each part of the body is concocted by the 

 heat in each part proper to it ; and if this heat is 

 unable to do its work the part suffers damage, and 

 deformity or disease is the result. A more detailed 

 account of this cause will have to be given in the 

 treatise Of Growth and Xutrition.^ In those persons 



* Not extant. 



527 



