96 The Potatoe Plague. 
of climate, soils or manures; warm or cold latitudes ; in hot, 
dry, wet, and cold seasons ; under every possible feature of 
cultivation, and in every condition of the crop, as managed by 
all sorts of farmers, throughout the whole of this country and 
all the countries of Europe. These facts remembered we 
must discard the idea that atmospheric influences would pro- 
duce the disease; manures and soils could not produce it; 
excessive heat or cold could not affect the crop so universally. 
The disease has been steadily increasing for years in defiance 
of all the conditions that these various theories would estab- 
lish as governing the malady. They are partial in their 
operation, and must, therefore, be rejected as insufficient. 
The cause of rot, says an intelligent writer, cannot be in the 
soil, since we find healthy and diseased potatoes growing be- 
side each other; that is to say, on soils of the same constitu- 
tion we sometimes find the rot in alternate plants, or in whole 
rows. It cannot be attributed to the atmosphere, as all plants 
and roots are equally surrounded by it ;-nor can the cause be 
in the manure, as all the tubers receive the same kind, and 
nearly the same quantity. It must, therefore, be attributed 
to the potatoe rtself. 
We have, then, to consider the most popular theory, which 
ascribes the disease to the influence of the growth of fungi. 
This position is the one taken by the principal vegetable 
physiologists of Great Britain and this country, and most of 
the directions published in the agricultural papers are made 
with reference to this fact. That this is not the true cause 
of the disease, is, I think, made sufficiently clear by the con- 
cluding paragraphs of the last chapter. That evidences of 
the appearance of fungi have been discovered, and are dis- 
coverable in all diseased potatoes, I do not deny, but I assert 
that they are a consequence of disease and not the cause. 
Liebig says in his Chemistry of Agriculture — “The micro- 
scopical examination of vegetable and animal matter, in the 
