5H CHAINS OF CIRCUMSTANCE 



chain of circumstances had been completed and had become 

 effective, were the separate links of the chain discovered. 

 Man drained the marshes and strange to say, one of the 

 most serious of the diseases of his domestic stock- the Liver- 

 rot of Sheep, began to disappear. At its worst, liver-rot 

 caused a mortality of three million sheep in England in a 

 single winter, that of 1879-80, in that of 1830-31, more 

 than two million are said to have succumbed, and Youatt, 

 writing in 1837, considered that the average number of 

 deaths amounted to more than one million sheep and lambs 

 every year. The older writers recognized that the disease 

 was confined to wet seasons, or followed upon pasturing 

 sheep upon ground moist and marshy at all seasons, but 

 who could have guessed the strange stages that led to the 

 reduction of the rot in Britain ? Many years of research 

 were necessary before the links in the chain were appreciated. 



The rot was caused by a flat-worm the " Liver-fluke,' 

 Fasciola (Distomum) hepatica which choked the bile-ducts, 

 and to the number of 870 has been found in the liver of a 

 single sheep. But the liver-fluke has a curious life-history : 

 the eggs develop in water, 'and the wriggling larva which 

 hatches from each, dies if it cannot find a Pond Snail 

 (Limn&a truncatula); within which it passes through suc- 

 cessive stages of development, until, reaching an active stage, 

 it leaves the snail, and forsakes the water to wriggle up any 

 convenient blade of grass. Here it comes to an untimely 

 end if a sheep does not happen to eat the grass; but if a 

 sheep be so unfortunate, the wheel of life is set in motion 

 again, and an adult liver-fluke develops which finds its way 

 to the bile ducts, and gives rise to the deadly liver- rot. In 

 no other way can the liver-fluke come into being, the water 

 and the Pond Snail are as necessary for its development as 

 the sheep itself. 



These then are the links in sequence which man un- 

 wittingly set in motion : Man drained the marshes, and so 

 doing he destroyed the Pond Snails, and so doing he broke 

 a link in the life-history of the liver-fluke, and destroyed the 

 parasites ; so sheep were freed from a scourge which 

 wholly destroyed many flocks, and in a manner threatened, 

 as Youatt says, to extirpate the breed of sheep in Britain. 



Yet, to be effective a recoil on man's prosperity need not 



