PROGRESSION OF ANIMALS, ix.-x. 



form were not greater than the one," movement would 

 be impossible for animals which advance by undula- 

 tions. For, when the bend is extended, they would 

 not have made any advance, if it subtended an equal 

 Hne ; whereas, in fact, it is longer when it is extended, 

 and then, when this part has come to a standstill, 

 the animal draws up the rest. 



In all the above-mentioned changes that which 

 moves advances by first extending itself straight out 

 and then curving itself — straightening itself out with 

 its leading parts and curving itself in the parts which 

 follow. All animals, too, which jump make a bend 

 in the lower part of their body and jump in this 

 manner. Animals also which fly and those which 

 swim, fly by straightening and bending their wings 

 and swim with their fins, some fish having four 

 fins and others, namely those which are of a more 

 elongated form (eels for example), having two fins. 

 The latter accomplish the rest of their movement 

 by bending themselves in the rest of their body, as a 

 substitute for the second pair of fins, as has already 

 been said. Flat-fish use their two fins, and the flat 

 part of their body instead of the second pair. Fish 

 that are entirely flat, like the ray, manage to swim by 

 using their actual fins and the outer periphery of their 

 body, which they alternately straighten and bend. 



X. A question might perhaps be asked as to how 

 birds, whether flying or walking, can move at four 

 points, in view of the statement that " all red-blooded 

 animals move at four points." But this is not exactly 

 what we stated ; what we said was " at not more than 

 four points." However, they could not fly if their 



tb.ese two lines together must be longer than the line which 

 subtends their angle. 



r2 515 



