MR. C. REID ON THE NATURAL HISTORY OF ISOLATED PONDS. 285 
tlie mud below. Observations on the species which inhabit a pond 
in successive years have still to be made. It is only in a large 
railed-in pond with shelving margins and tolerably deep water in 
the centre, that we may expect to obtain clear evidence of the 
rapidity with which the introduction of species takes place, and it 
is certainly in such ponds that the variety is greatest. 
Although my notes have been mainly confined to ponds which 
are certainly no older than the present century, yet so considerable 
a proportion of our aquatic flora, and of our freshwater mollusca, 
has been observed in these ponds, that one is inclined to say that 
with longer time for the accumulation of the results of rare 
accidents, most of the other species would appear also. We know, 
for instance, that some of our bivalves are transported attached to 
the toes of birds, and wo know that these species are found in 
various natural lakes, though so rare in modern ponds. Is not the 
common occurrence of these species in natural lakes merely the 
result of time? In the one case wo arc dealing with a pool of 
water, the whole history of which is included in the present century ; 
in the other we seo a larger lake whose origin dates back probably 
ten thousand years. 
No doubt it will be said that the eggs of many aquatic animals, 
and the seeds of many water-plants are not fitted for transport by 
birds. Hut even with these, the eggs or seeds of most must 1x3 
occasionally included in the mud which adheres to the feet of birds, 
or they must sometimes be entangled in the floating plants which 
so readily are transported. Add to this, that when raptorial birds 
were more abundant, the -violent death of so many of the smaller 
species must often have led to the scattering of the undigested 
contents of their stomachs, and thus to the occasional introduction 
into river-basins of animals and plants previously unknown there. 
The natural history of isolated ponds shows that when results 
are accumulated during long periods, accidents apparently rare 
may have more to do with the distribution of animals and plants 
than is imagined. Among naturalists there is a tendency to 
account for isolated colonies by former continuity in nearly every 
case. We are told that the botanical and zoological similarity of 
our different river-basins is due to the former connection of the 
rivers. In the case of the British rivers, however, we must guard 
against any such conclusion. Each year’s work at the subject 
