GENERATION OF ANIMALS, II. viii. 



that is wanting to make it such is that its secretion 

 should be colder, and this occurs when it is united 

 with that of the ass. In the same way the ass comes 

 ^vithin an ace of generating infertile offspring even 

 when it mates with its owti kind ; so that when there 

 is the additional factor of unnatural mating beside 

 the difficulty it has in producing even a single young 

 one in the normal way, the resultant offspring is still 

 more infertile and unnatural ; in fact, it will lack 

 nothing to make it completely infertile, and ^\1ll be 

 infertile of necessity. 



Furthermore, female mules grow large in size. 

 This is because the secretion intended for the men- 

 strual flow is diverted to produce growth. And 

 since the period of gestation in such animals lasts 

 a year, the female mule not only has to conceive but 

 has to nourish the embryo all that time ; and this is 

 impossible unless menstrual flow is being produced. 

 None is produced in mules : the unserviceable part 

 of the nourishment is passed out together \vith the 

 residue that comes from the bladder (which explains 

 why male mules do not smell at the pudenda of the 

 females as the other solid-hoofed animals do, but at 

 the residue itself) ; the rest of the nourishment is 

 diverted to growth of the body and to size. Hence 

 although it is possible for the female to conceive 

 occasionally — and indeed the fact is estabhshed that 

 this has happened — it is impossible for her to nourish 

 an embryo for the full period and bring it to the birth. 

 The male may occasionally generate (a) because " the 

 male is by nature hotter than the female, and (b) 

 because the male does not contribute any corporeal 



and are cited here to explain how the male mule may be able 

 to generate. 



259 



