president’s address. 
479 
the sea may have been successfully effected. Where, for 
example, salt lakes exist at a reasonable distance from the sea, 
it might be possible for a littoral marine animal transported to 
them by any agency to make good its footing. Probably the 
Copepod, Wolterstorffia blanchardi, which occurs in the 
Algerian Sahara, has been introduced in this way, and it has 
now spread from highly salt water to other pools which, 
though always of a relatively high salinity, are yet properly 
called fresh. 
Apart from such special cases, transportation can have 
played no part in the colonisation of fresh waters. 
2. Relict Lakes. 
In many parts of the world, owing to geological changes 
or other causes, parts of the sea have become isolated from the 
rest, while retaining drainage outlets. In course of time the 
water of these basins has become fresh, so that lakes have 
taken the place of sea, and such lakes are known as “ Relict 
Lakes.” Or such lakes have arisen by the shrinking of an 
ancient sea and the freshening of its waters, as has happened in 
the case of the Oligocene Mainz basin and the Caspian Sea. 
In either case the marine fauna of the area so isolated would 
find itself faced by the alternative of either adapting itself to 
live in fresh water or else succumbing, and it is supposed that 
in this way many marine species have become adapted to fresh 
water. 
There is conclusive geological, and sometimes historical, 
evidence that many lakes owe their existence to causes of this 
kind, but there are comparatively few which contain animals 
which are, in fact, relics of their original marine fauna. 
The presence in lakes of species of marine origin has often 
been accepted as conclusive evidence of the relict nature of 
such lakes, whereas it can at most be regarded as confirmatory 
of evidence drawn from geology. There are fundamental 
fallacies inherent in the use of such zoological evidence. In 
the first place, the marine relationship of a species found in fresh 
water is open to discussion. While, for example, the presence of 
