GENERATION OF ANIMALS, V. v. 



tion. Of all the animals known to us, it is most 

 marked in the horse, the reason being that in the 

 horse the bone which surrounds the brain is, in pro- 

 portion to the animal's size, thinner than that of any 

 other animal. A proof of this is that a blow deUvered 

 on this spot is fatal to a horse. Homer's lines " fit in 

 with this too : 



Where on a horse's skull his hairs first grow. 

 And where he suffers his most fell aild fetal blow. 



Therefore, since the thinness of the bone makes it 

 easy for the stream of fluid to flow to the hair at this 

 place, and as the heat begins to fail on account of 

 age, the result is that this hair goes grey. Reddish 

 hair goes grey more quickly than black, as redness 

 too is a sort of infirmity of the hair, and everything 

 that is weak ages more quickly.^ Cranes, however, 

 so it is alleged, go darker as they get older. If this 

 allegation is true, the reason for this condition would 

 be that the nature of their feathers is more fluid, and 

 that as the birds grow old the fluid in their feathers 

 is too plentiful to putrefy easily.*^ 



Here are proofs (a) that greyness is produced by 

 putrefaction of some sort, and (b) that it is not, as 

 some people imagine, a process of withering. Proof 

 of (a). Hair that is protected by hats or other cover- 

 ings goes grey more quickly, the reason being that 

 the effect of the wind blowing is to prevent putrefac- 



" niad VIII. 83-84. 



* See 775 a 19 ff. 

 ' See above, 785 a 2. 



5SS 



