PARTS OF ANIMALS, IV. xii. 



duced alive or out of eggs, have an umbilicus while 

 they are developing, but when they are more fully 

 grown it ceases to be visible. The reason for this is 

 clear from what happens during their development : 

 the umbilical cord grows on to the intestine and 

 unites with it, and does not form a part of the system 

 of blood-vessels, as it does in the Vivipara." 



The good fliers have big strong wings, e.g. the 

 birds which have crooked talons and feed on raw 

 meat : these must be good fliers owing to their habits 

 of life, and so they have an abundance of feathers and 

 big wings. But there are other sorts of birds which are 

 good fliers beside these : birds Avhose safety lies in 

 their speed of flight ; and migrants. Some birds are 

 poor fliers : heavy birds, which spend their time on 

 the ground and feed on fruits ; or birds that live on 

 and around the water. The crook-taloned birds, leav- 

 ing out of account their wings, have small bodies, be- 

 cause the nutriment is used up to produce the wings 

 and weapons of offence and defensive armour. The 

 poor fliers, on the contrary, have bulky, and therefore 

 heavy, bodies. Some of these instead of wings have 

 as a means of defence " spurs " on their legs. The 

 same bird never possesses both spurs and talons, and 

 the reason is that Nature never makes anything that 

 is superfluous or needless. Spurs are of no use to a 



states that in the bird's egg, as the embryo grows, the allantois 

 (the " second umbilicus ") collapses first and then the " first 

 umbilicus" {Be gen. an. 754 a 9). Actually the reverse 

 order is the correct one, but the interval is comparatively 

 short. The umbilical vesicle in mammals, which shrivels 

 very early in the process of development, escaped the notice 

 of Aristotle, who supposed their allantois to be comparable to 

 the umbilical vesicle of reptiles and birds. The umbilical 

 vesicle of mammals was discovered by Needham in 1667. 

 (See Ogle's note ad loc.) 



409 



