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- 



