PROGRESSION OF ANIMALS 



I. We must next discuss the parts which are useful 

 to animals for their movement from place to place, 

 and consider why each part is of the nature which 

 it is, and why they possess them, and further the 

 differences in the various parts of one and the same 

 animal and in those of animals of different species 

 compared with one another. We must first decide 

 what questions we have to discuss. 



One question is, what is the smallest number of 

 points at which animals move ; the next is, why red- 

 blooded animals move at four points, while bloodless 

 animals move at more than four ; and, in general, 

 why some animals are without feet, others biped, 

 others quadrupeds, and others polypods, and why all 

 that have feet at all have an even number of feet ; 

 and, in general, why the points at which movement 

 is made are even in number. 



We must further consider why a man and a bird are 

 bipeds, while fishes are without feet ; and why a man 

 and a bird, being both bipeds, have opposite bend- 

 ings of the legs. For a man bends his legs in a convex 

 direction, a bird in a concave direction ; and a man 



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