372 
THE FKESH- WATER LOCHS OF SCOTLAND 
small mussel which appears to have spread enormously within recent 
years in our European river-systems. By the byssus, which is 
characteristic of these forms, it has attached itself to ships and rafts, 
and so procured transport from place to place. It should be noted 
that Dreissensia still retains a free-swimming larva, which thus secures 
the distribution of the species through all parts of a river below that 
to which the adult has been carried. 
We have already referred to dispersal by the agency of wind. 
There is no doubt that this is most important in the case of small 
invertebrates which are able to encyst themselves, and in the case of 
those forms producing ova which can resist desiccation. Certain 
Rotifers, a species of Cyclops^ and some Protozoa come into the 
former category, but into the latter comes a much greater number of 
types. We include the gemmules and statoblasts of sponges and 
Polyzoa, the summer eggs of various Entomostraca, and the horny- 
cased eggs of Hydra and the Rotifers. No doubt we may add to the 
list the seeds and spores of diverse aquatic plants. 
The matter is peculiarly interesting in the case of Rotifers, which 
often appear in sporadic fashion in widely separated areas thousands 
of miles apart. We are as sure as we can be of anything in this 
somewhat speculative domain, that this remarkable discontinuous 
distribution is due to the transport of the ova (in some cases perhaps 
the encysted adults) by means of wind. The eggs are very minute 
bodies, few of them exceeding a three- hundredth of an inch in length, 
and many considerably smaller, so that they are specially adapted for 
transport with dust by the aerial currents which circle the globe. 
The finding of isolated specimens in remote districts, more striking 
amongst the Rotifers than in the case of most other animals, is a 
direct index of the minuteness of their resistant ova, which affords 
special facilities for wind-transport. 
One other means may be mentioned as occasionally effecting the 
dispersal of fresh-water organisms, and that is the agency of floods. 
In flood-time barriers of river-systems may break down ; but without 
going as far as this, we may conceive of local varieties, or even species, 
being swept from their point of origin in some backwater, and widely 
distributed within the same river-basin. Isolated ponds and lakes 
may receive in times of flood many organisms from a distance, or on 
the other hand they may have peculiar and characteristic forms 
carried out of them by the overflowing waters. 
Enough has now been said to make it clear that the organisms 
which actually constitute the present-day fresh-water flora and fauna 
are unceasingly subject to dispersal within the limits of that medium, 
by a variety of different means. This being so, what is more likely 
than that these organisms should assume and retain a uniformity 
