MR. C. REID ON TI1E NATURAL HISTORY OP ISOLATED PONDS. 275 
outlet and the water evaporates, or the only outlet is by percolation. 
Such ponds are perfectly isolated, and though some of them may 
disappear during exceptionally dry summers, the majority are 
perennial, and always contain sufficient water to keep alive the 
animals and plants which inhabit them. 
It might be thought that an isolated pond on an open Down, a 
mile or more from the nearest stream, and perhaps 400 feet above 
its level, would contain no life except the aquatic larval of flying 
insects, a few creatures such as entomostraca, of which the eggs 
are small, can bear dessication, and might be transported by the 
wind, and certain plants with small spores or winged seeds. This, 
however, is not the case. Aquatic plants and animals are common 
under such conditions ; but among the more highly organised 
species the most common are not those which could be brought by 
the wind, they are species which are heavy, and which have seeds 
or eggs of considerable size and weight. 
Some fourteen years ago I commenced occasional observation of 
the higher animals and plants which were found in isolated ponds, 
but, unfortunately, have lost my Norfolk notes. In the present 
paper only the general results of the earlier work can therefore be 
given. For two reasons the observations have been confined to the 
amphibia, fish, mollusca, and flowering plants. In the first place, 
I have not much time at my disposal ; and secondly, the other 
invertebrate, and the cryptogams, are more easily transported, and 
their occurrence would, therefore, be of less importance for the 
present enquiry. 
Though it will be needless to trouble the Society with a mass of 
details, it may be advisable before giving general results to quote 
a few examples, to show the method employed, and the character 
of the evidence. I will take for this purpose the district most 
recently examined ; a district which happens also to be the one 
where the ponds are most completely isolated. 
The South Downs form a range of undulating chalk hills, 
extending through Sussex from Beachy Head to the Hampshire 
boundary. They constitute a district pre-eminently dry ; no 
streams occupy the coombes, and the only water visible is in the 
few rivers which, rising in the Weald, must cut through the 
Downs to reach the coast. All the rainfall of this district either 
evaporates or sinks into the ground without forming streams, 
