284 MR. C. REID ON THE NATURAL HISTORY OF ISOLATED PONDS. 
But the Water-lilies, the fruit of which we know are eaten by 
Coots, do not appear in isolated ponds. 
Perhaps the occurrence of the fruitless Elodea, and the fact that 
most of the plants found in distant ponds have, like Elodea , 
extremely brittle stems, detached fragments of which will grow, 
may give the clue to the ordinary mode of dispersal. All these 
plants, besides brittle stems, have finely divided or thin leaves, 
which, on removal from the water, collapse and cling closely to any 
object which they may touch. It is therefore not improbable that 
most of them are transported in fragments, which breaking, cling 
to the feet of waders, to be washed off when the bird flies to the 
next pond. If fragments of stems are carried by birds, this will 
also account for the constant occurrence of the Limnseids, which 
both adhere to the stems and attach their eggs to them. The other 
freshwater mollusca, except the small Pisidia, mostly live in the 
mud, are viviparous, or do not attach their eggs to plants ; for 
these reasons the operculatcd mollusca, and the larger bivalves, are 
less likely to find their way to isolated ponds. 
Although so many aquatic animals and plants have now been 
added to the list of species which can cross barriers of dry land, we 
are still in great danger of under-estimating the rapidity with 
which the dispersal takes place. As yet only a limited number of 
observations on isolated ponds have been made ; and these ponds 
are small, liable to shrink greatly in dry weather, and are usually 
exposed to the trampling of large animals which come to drink. 
Thus the species found, as already remarked, may represent most 
imperfectly the number that have been carried to each pond since 
its formation. In most cases only one or two individuals would be 
transported, and the species for the first year or two would be 
particularly liable to extermination, on account of the small 
number. In a small pool of water competition also must be very 
active, and must tend to keep down the number of species living 
together, especially when many of the forms have similar habits, as 
is the case with the Limnseids. We might, therefore, expect to 
find in the mud of the pond the remains of various species which 
had been introduced at different times, had lived and multiplied, 
and had then been crowded out by stronger competitors. The 
species found in any one small pond may be merely the victors in 
the struggle ; the remains of the conquered races will be found in 
