FUNGI. ' 407 
Helena by the north-east trade-winds, which bring 
from the same continent those singular red dust 
clouds which the microscope of Ehrenberg found 
to be composed of vegetable organisms, and which 
have served as tallies upon the viewless winds, 
indicating the course of their currents. St. Helena 
lies in the same latitude with Peru, and is nearer 
the native habitat of the potato than any other 
country in which the disease has been subse- 
quently experienced. In this island, finding the 
conditions of moisture and temperature favourable 
to its development, it increased with amazing rapi- 
dity, loading the air with myriads of its impalp- 
able seeds. Thence it seems to have been carried 
by the winds to Madeira and North America ; and 
so has progressed from country to country, gain- 
ing new accessions of strength and numbers from 
every field, speedily making its dread presence 
known wherever it alighted. It reached England 
in the autumn of 1844, and seems at first to have 
been confined exclusively to the south-western 
districts. From Kent it travelled west and north, 
devastating the whole of Ireland and halting mid- 
way in the South of Scotland, so that the crops 
in the Highlands were that year free from the 
pest. In 1846 it proceeded throughout the north 
of Scotland, where its effects in certain districts 
were scarcely less disastrous; thence on to the 
