BOOK III 



We have spoken about the sterihty of mules, and I 

 about the animals which are vi\iparous both exter- animau^^ 

 nally and internally. We now pass on to those (continued;. 

 blooded animals which are oviparous. The pheno- "P"* • 

 mena of generation here are on the one hand similar 

 to those which obtain in the animals that walk," so 

 that the same statement will serve for all of them ; 

 on the other hand, these animals exhibit certain 

 differences not only as between themselves, but also 

 when compared ^vith the animals that walk. Their 

 generation is the result of copulation, i.e., of the 

 emission of semen into the female by the male : this 

 appUes to all of them, of course. But beyond that 

 there are variations : (a) Birds produce a perfect * 

 egg with a hard shell (unless it be deformed by 

 disease). All birds' eggs are of two colours, (b) The 

 Selachian fishes, as I have often repeated, are inter- 

 nally o\'iparous but bring forth their young aHve, 

 after the egg has moved from one position in the 

 uterus to another. Their egg is soft-shelled and of 

 one colour only.*^ The fish known as the fishing- 

 frog '^ is the only one in this class that is not internally 

 viviparous. The cause of this will have to be stated 

 later.* (c) All other fishes that are oviparous pro- 



■* Probably Lophius piscatorius ; see 754 a 26, n. 

 • At 754 a 25-31- 



263 



