PROGRESSION OF ANIMALS, xiv.-xv. 



not move separately with their front and back legs'*; 

 and, if they moved with both their right legs first, 

 they would not be above their supporting hmbs and 

 would fall. If, then, they must necessarily move in 

 one or other of these two ways or else diagonally, 

 and neither of the first two ways is possible, they 

 must necessarily move diagonally ; for if they move 

 thus they cannot, as has been explained, suffer 

 either of the above ill results. For this reason horses 

 and similar animals stand at rest with their legs 

 advanced diagonally and not ^^-ith both right or both 

 left legs advanced at the same time. And those 

 animals which have more than four legs move in 

 the same manner ; for in any four adjoining legs the 

 back legs move diagonally with the fore legs, as can 

 be plainly seen in those which move slowly. 



Crabs too move in the same fashion, for they are 

 among the polypods. They, too, always move on 

 the diagonal principle in whatever direction they are 

 proceeding. For this animal moves in a pecuUar 

 manner, being the only animal to move obliquely 

 and not forward. But since " forward " is determined 

 in relation to the vision, nature has made the crab's 

 eyes able to conform with its limbs ; for its eyes 

 move obliquely, and so, for this reason, crabs too can, 

 in a sense, be said to move ** forward." 



XV. Birds bend their legs in the same manner as 

 quadrupeds ; for in a way their nature is closely 



" i.e. do not move first the front legs together and then 

 their back legs together. The ms. authority is strongly in 

 favour of the omission of the negative: but 712 b 4 " one or 

 other of these two ways " implies the alternative of movement 

 with the front legs together and then the back legs together, 

 or else with the right legs together and then the left legs 

 together. 



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