GENERATION OF ANIMALS, II. viii. 



cold quarter. For this reason they do not put the jack- 

 asses to the females at the equinox, as is done with 

 horses, but at the time of the. summer solstice, so that 

 the asses' foals may be bom when the weather is 

 warm. (Since the period of gestation in both horse 

 and ass is a year, the young are born at the same 

 season as that when impregnation takes place.) As 

 has been said, the ass is by nature cold ; and a cold 

 animal's semen is, of necessity, cold like itself. (Here 

 is a proof of it. If a horse mounts a female " which 

 has been impregnated by an ass, he does not destroy 

 the ass's impregnation ; but if an ass mounts her 

 after a horse has done so, he does destroy the horse's 

 impregnation — because of the coldness of his own 

 semen.) Thus when they unite \\'ith each other, the 

 impregnation remains intact by reason of the heat 

 resident in one of the two, \iz., that of the horse, 

 whose secretion is the hotter. Both the semen from 

 the male and the matter supplied by the female are 

 hotter in the case of the horse ; with the ass, both 

 are cold. So when they unite — either the hot one 

 added to the cold, or the cold added to the hot — the 

 result is (a) that the fetation which is formed by them 

 continues intact, i.e., these two animals are fertile 

 when crossed with each other, but (b) the animal 

 formed by them is not itself fertile, and cannot pro- 

 duce perfect offspring. 



Besides, both horse and ass have a general natural 

 disposition to be infertile. I have already mentioned 

 several points about the ass, and another is that 

 unless it begins to generate after the first shedding 

 of teeth, ** it never generates at all ; so close does the 

 ass come to being infertile. It is the same >\ith the 

 horse ; it is naturally disposed to be infertile ; all 



K 257 



