BOOK V 



We must now study the " conditions " in respect I 

 of which the parts of animals differ. I mean such (^^^ndary 

 conditions of the parts as the following : blue and sex-char- 

 dark colour of the eyes, high and deep " pitch of the 

 voice, and differences of colour and of hair or feathers. 

 Some of these conditions are found throughout cer- 

 tain classes of animals ; some occur irregularly, and 

 a striking instance of this is afforded by the human 

 species. Further, there are some conditions, accom- 

 panying the changes in the times of Ufe, which occur 

 in all animals alike, but there are others which are 

 divergent in different animals, as, e.g., those which 

 have to do with the voice and the colour of the hair : 

 thus, some animals do not go noticeably grey towards 

 old age, whereas man is affected by this condition 

 more than any other animal. Again, some of these 

 conditions come on immediately after birth, others 

 make themselves noticed as age advances, or in old 

 age. When we come to consider these conditions 

 and all others like them, we must not supp)ose that 

 the same sort of cause is operative as before, for 

 there are certain conditions which are not character- 

 istics belonging to Nature in general, nor pecuharities 

 proper to this or that particular class of animal ; and 

 whatever the quaHty of such conditions may be, in 



• See 787 b 1, n. 



485 



