408 FIRST FORMS OF VEGETATION. 
Shetland and Faroe Islands, and to northern 
latitudes, as far as the limits of the cultivation 
of the potato in that direction extended. On 
the Continent it has been observed to pro- 
gress in a similar manner ; its geographical limits, 
as well as its intensity, becoming more ex- 
tended and marked with each succeeding year. 
Why the fungus should have been introduced in 
1845 and not in previous years, and why it should 
then all at once have acquired such fearful power, 
we cannot positively tell, any more than we can 
tell why the memorable plague of London, or 
those deadly pestilences which swept over Europe 
in the middle ages, should have sprung up so sud- 
denly as they did. Almost all authorities are 
agreed that the disease generally makes its first 
decided appearance during thundery weather; 
and the exceptional amount of electrical disturb- 
ance which extended over the whole kingdom 
during the summers of 1845 and 1872, seems to 
have been most unfavourable to the potato-crop. 
But these were not the only wet and thundery 
summers that have been known in Britain since 
the introduction of the potato; and the climatal 
conditions of the various countries in which the dis- 
ease appeared in 1845 were as different as possible. 
It has therefore been suggested that cosmical 
conditions may at definite intervals favour the 
