Chap. III. THEIR VARIATION. 97 



rinary surgeon, remarks, "few or none die of the rot, but they 

 are phthisical ; not one of them from a torrid climate lasts out 

 the second year, and when they die their lungs are tuber- 

 culated." 87 Even in certain parts of England it has been found 

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

 the banks of the Ouse, the Leicester sheep were so rapidly de- 

 stroyed by pleuritis 88 that the owner could not keep them; the 

 coarser-skinned sheep never being affected. 



The period of gestation was formerly thought to be so un- 

 alterable a character, that a supposed difference between the 

 wolf and the dog in this respect 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, 89 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 150-3 days. 



Southdowns 144-2 



Half-bred Merinos and Southdowns . . 146-3 



f blood of Southdown 145-5 ' 



s » >, 144-2 " 



In this graduated difference, in these cross-bred animals having 

 different proportions of Southdown blood, we see how strictly 

 the two periods of gestation have been transmitted. Nathusius 

 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 South- 

 downs 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 birft, of which fact the 

 curious Shangai sheep (with their truncated and rudimentary 



8 '" Youatt on Sheep, note, p. 491 in < r> „ „ 



*> < The Veterinary,' vol. x p. 217. £ S ^o^ d ' Acclimat -'' tom - 



S9 A translation of his paper is given ' ' P " 



VOL. I. 



H 



