Chap. in. THEIR VARIATION. 10] 



possible to keep certain breeds of sheep ; thus on a farm on 

 the banks of the Oiise, the Leicester sheep were so rapidly 

 destroyed by pleuritis 89 that the owner could not keep them ; 

 the coarser-skinned sheep never being affected. 



The period of gestation was formerly thought to be of so 

 unalterable a character, that a supposed difference of this kind 

 between the wolf and the dog was esteemed a sure sign of 

 specific distinction ; but we have seen that the period is 

 shorter in the improved breeds of the pig, and in the larger 

 breeds of the ox, than in other breeds of these two animals. 

 And now we know, on the excellent authority of Hermann 

 von Nathusius, 90 that Merino and Southdown sheep, when 

 both have long been kept under exactly the same conditions, 

 differ in their average period of gestation, as is seen in the 

 following Table : — 



Merinos 1503 days. 



Southdowns 1442 „ 



Half-bred Merinos and Southdowns .. 146"3 „ 



f blood of Southdown 1455 „ 



i „ ,, 144-2 „ 



In this graduated difference in cross-bred animals having 

 different proportions of Southdown blood, we see how strictly 

 the two periods of gestation have been transmitted. Nathu- 

 sius remarks that, as Southdowns grow with remarkable 

 rapidity after birth, it is not surprising that their foetal 

 development should have been shortened. It is of course 

 possible that the difference in these two breeds may be due 

 to their descent from distinct parent-species; but as the 

 early maturity of the Southdowns has long been carefully 

 attended to by breeders, the difference is more probably the 

 result of such attention. Lastly, the fecundity of the several 

 breeds differs much ; some generally producing twins or even 

 triplets at a birth, of which fact the curious Shangai sheep 

 (with their truncated and rudimentary ears, and great Eoman 

 noses), lately exhibited in the Zoological Gardens, offer a 

 remarkable instance. 



Sheep are perhaps more readily affected by the direct action 



89 'The Veterinary,' vol. x. p. 217. given in ' Bull. Soc. Imp. d'Accliinat./ 

 ■° A translation of his paper is torn, ix., 1862, p. 723. 



