GENERATION OF ANIMALS, I. iv.-v. 



the movement of the seminal residue more steady. 

 (1) In the ^'i\ipa^a, as for instance in horses and 

 other such animals, and also in man, they do this by 

 maintaining in position the doubling-back of the 

 passages (for a description of this reference must be 

 made to the Researches upon Animals),'^ since the 

 testes are no integral part of the passages : they are 

 merely attached thereto, just like the stone weights 

 which women hang on their looms when they are 

 wea\-ing. ^ When the testes are removed, the passages 

 are drawn up within ; this is why castrated animals 

 cannot generate, whereas if the passages were not 

 so drawn up they would be able to do so. A bull im- 

 mediately after castration has been kno\\'n to mount 

 a cow and effect impregnation,"^ because the passages 

 had not yet been drawn up. (2) In Birds and in the 

 oviparous quadrupeds the testes receive the seminal 

 residue, so that its emission is slower than it is in the 

 case of Fishes.'' This is clearly to be seen in Birds : 

 their testes are much larger at the time of copula- 

 tion.* Those birds which copulate at one season only 

 of the year have such tiny testes when this period is 

 over that they are almost indistinguishable, whereas 

 during the breeding season they are very big. So 

 then the animals whose testes are internal accom- 

 plish their copulation more quickly, since in fact those 

 with external testes do not emit the semen until the 

 testes have been dravvn up. 



Another point. The organ for copulation is present V 

 in the quadrupeds because it is possible for them to 

 have it, whereas it is not possible for birds and foot- 



" H.A. 310 a -20 ff., and 718 a 15 below. 



* Cf. 787 b 2Q. ' Cf. H.A. 510 b 3. 



'' Which have no " testes " in Aristotle's sense. 



* Cf. H.A. 509 b 35 flF. 



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