325 
one of the probable reasons for this was stated in 
the last Lecture. 
The most remarkable instance of the powers of 
vegetables to exhaust the soil of certain principles 
necessary to their growth is found in certain fun- 
guses. Mushrooms are said never to rise in two 
successive seasons on the same spot, and the pro- 
duction of the phenomena called fairy rings has 
been ascribed by Dr. Wollaston to the power of the 
peculiar fungus which forms it to exhaust the soil 
of the nutriment necessary for the growth of the 
species. The consequence is, that the ring annu- 
ally extends ; for no seeds will grow where their 
parents grew before them ; and the interior part 
of the circle has been exhausted by preceding 
crops ; but where the fungus has died, nourishment 
is supplied for grass, which usually rises within the 
circle, coarse, and of a dark green colour. 
When cattle are fed upon land not benefited by 
their manure, the effect is always an exhaustion of 
the soil ; this is particularly the case where carry- 
ing horses are kept on estates ; they consume the 
pasture during the night, and drop the greatest 
part of their manure during their labour in the 
day-time. 
The exportation of grain from a country, unless 
some articles capable of becoming manure are 
introduced in compensation, must ultimately tend 
to exhaust the soil. Some of the spots now desert 
sands in northern Africa, and Asia Minor, were 
anciently fertile. Sicily was the granary of Italy : 
and the quantity of corn carried off from it by the 
Romans, is probably a chief cause of its present 
y 3 
