i 
274 MR. C. REID ON THE NATURAL HISTORY OF ISOLATED PONDS. 
both more sweeping and more rapid than is thought, have led me 
into various by-paths. Especially have I been led to study the 
“ outliers,” which are completely separated from other favourable 
stations by tracts of desert or unsuitable country, and yet contain 
the same species. Various classes of these “oases” are to be 
found. We have the fertile oasis in the desert of Sahara ; the 
isolated mountain, bearing an arctic flora in a temperate region ; 
the outlier of limestone, with its peculiar fauna and flora; the 
isolated pond ; and other varieties of oasis too numerous to 
mention. 
One of my enquiries has been, How do the animals and plants 
of isolated ponds get there, and how long do aquatic species take 
to spread across barriers of dry land to a newly formed basin'? 
The bearing of the enquiry is this : If in completely isolated 
artificial ponds we constantly find aquatic plants and animals, not 
brought by man, and if we can prove that these ponds had no 
existence till recent times, then it follows that such species have 
great facilities for dispersal, and can be transported across a belt 
of uncongenial country. This will be true, even though up to the 
present time we have not traced the actual transfer of the species, 
and do not yet know by what means they are carried. The more 
common the occurrence of any species in isolated ponds, the 
greater, as a general rule, must be its facilities for dispersal ; and 
also, the less will it prove any former water-connection with other 
districts, or show the long continuance of suitable conditions. 
The ponds to which the following notes are confined form a 
somewhat peculiar class. I do not propose to refer to any of the 
lakes, broads, or meres, for all of these have some outlet up which 
aquatic species might have spread from other districts. The 
mountain tarns also will not be dealt with, for though in many of 
them the only outlet is over a waterfall, and they are truly isolated, 
I have not yet had sufficient opportunities for studying their 
natural history. 
Two classes of ponds remain — the isolated pool of water that 
collects in an old brick-yard, quarry, or marl or gravel-pit, and the 
“ dew-pond,” which is dug on the dry Chalk Down to provide 
water for the cattle and sheep. The only supply in both these 
cases is received from the rain or dew which falls in the immediate 
neighbourhood, or percolates through the soil ; there is either no 
