276 MR. C. REID ON THE NATURAL HISTORY OF ISOLATED PONDS. 
except in. two or three of the deepest valleys, and these only 
contain water in part of their course, and at certain seasons. 
The dryness of this region has made it absolutely necessary to 
provide water for the cattle by digging ponds at frequent- intervals. 
The bottom of these ponds is made impervious by puddling with 
clay or clialk-mud, or sometimes by a lining of concrete. When 
once formed the rain, dew, and condensation of the mists, which 
often hang on the tops of these hills, suffice to provide a constant 
supply of water, except, perhaps, during droughts like that of the 
summer of 1887. We may thus find ponds which are distant two 
or three miles from the nearest stream or marsh ; and as the Downs 
rise to 800 feet, and the average height is fully 200 feet above the 
highest streams, it follows that any aquatic animals or plants found 
in the ponds must have been transported up hill, as well as across 
uncongenial tracts of dry grass. 
Human agency is not likely to play much part in the transport 
of seeds or eggs to these ponds, for most of the Downs are seldom 
visited by strangers, and the majority of the ponds are not near 
any road. Open natural pasture, with an occasional pond, occupies 
much of the higher land, and on this, day after day, one meets 
only the shepherd tending the sheep, or the farmer making a short 
cut across the open country. There are no vehicles which might 
transport seeds in the mud adhering to their wheels, as may often 
have been the case with the plants found in horse-ponds. Even 
the spades which were used when the dew-pond was dug came 
from an upland farm which drew its supply of water either from 
a deep well or from an isolated pond ; so any earth sticking to the 
tools would only contain ordinary weeds of cultivation, not seeds of 
aquatic plants. The sheep remain on the Downs, and when taken 
to the lower lands to fatten they do not come back again ; when 
transferred to lower ground during severe weather they are kept 
as far as possible on dry spots. The shepherd lives not far off, 
and the mud on his boots is usually upland chalk-mud. Thus 
everything seems to show that human agency is not likely to 
transport any aquatic plants or animals to these dew-ponds. 
I will now give a few examples of the dew-pond and its natural 
history, premising that the distances and heights are taken from 
the six-inch Ordnance Survey, and that usually I could only stay 
to note the common and conspicuous plants and animals. It may 
