CH. VII.] CHANGES IN POND-WEEDS. 229 
The changes which occasionally take place in 
Pond-weeds are very striking, and apparently 
inexplicable—I have watched them with much 
interest in a chain of some eight ponds, all fed 
by the same stream, and occupying together a 
space of about one third of a mile in length. 
Until about eighteen or twenty years ago that 
sea-weed-looking nuisance the Potamogeton cris- 
pum was, I believe, unknown throughout the 
whole of this chain. Shortly after that time the 
second pond in it became almost covered with 
this weed, while the upper one was suffering 
from a scummy infliction (conferva) exclusively. 
The “potamogeton” pest seemed then to desert 
the second pond, and move upwards en masse, 
the scum, which had pervaded the upper pond, 
giving way to its more powerful rival, which 
completely filled it, while but one or two minute 
pieces of the weed were visible in the second. 
The ponds between the second and the last in 
the chain remained for some time uninoculated 
with the new weed, but a great part of the last 
has now become quite choked up by it, the inter- 
mediate ones remaining still almost entirely free 
from it, and the upper one being comparatively 
free both from it and the scum which had for- 
