404 FIRST FORMS OF VEGETATION. 
differences they exhibit in their qualities in dif- 
ferent countries. 
The first failure of the potato crop, which came 
like one of those sudden hurricanes of the tropics, 
carrying death and desolation in their train, is 
doubtless vivid in the recollection of all middle- 
aged people. This root, from its extraordinary 
productiveness, with little labour or exertion of 
any kind, became gradually a substitute in whole 
districts, especially in Ireland and the Highlands 
of Scotland, for the older cereal crops, as the 
staple food of the people; so that when a blight 
fell upon it, and the crop everywhere completely 
failed, hundreds of thousands were deprived of 
their sole means of subsistence, famine and its 
consequent malignant fevers rapidly spread 
throughout the land, and the social and agricultural 
system based upon this uncertain and narrow 
foundation was convulsed and completely broken 
up. Nor was this disease a temporary scourge ; 
it has returned every year since, with more or less 
fatality, so that the potato has become one of the 
most troublesome and precarious of all our crops. 
The year before last the potatoes were irredeem- 
ably bad, having failed to the extent of three- 
fourths of the yield. A minute mould, called 
Peronospora infestans (Fig. 49), consisting of grey 
interwoven filaments, bearing stalks some of 
