president’s address. 
483 
Crustacea, of which there are a great number of species, are 
clearly of fresh-water origin and show no evidence of a con- 
nection with the sea. The nature of the fauna with regard to 
mammals, fish, sponges, and worms, points with almost irresis- 
tible force to the conclusion that Lake Baikal must recently 
have been severed from the sea, and yet its situation and 
geological evidence so positively forbid such a conclusion that 
some other explanation of the peculiar fauna must be sought. 
(3) Immigration up Rivers. 
Where rivers run into the sea there is usually an estuary 
where the conditions are intermediate between those of fresh 
water and the sea. Within this intermediate region is found a 
fauna of a rather special nature, adapted to the brackish water 
and to the variable conditions. This brackish water fauna may 
penetrate far up the course of sluggish tidal rivers, and may 
even establish itself in water which is practically fresh. Our 
Norfolk rivers, the Yare and Bure, provide instances of such 
invasion. Species of the littoral genus Sphseroma have occured 
up the Yare in Rockland Broad, where the water is quite fresh, 
and in the Bure a variety of such animals have been taken at 
different times in fresh water. 1 To mention but a few, the 
Copepoda Eurytemora affinis and Tachidius littoralis are 
established inhabitants of the river and certain of the Broads, 
to which salt water rarely reaches, while Cordylophora, and 
together with it, Corophium volutator, have been found at 
Ludham Bridge, and the former in Barton Broad. 
The river road is always open to such animals as can over- 
come the obstacles that have been described, and it is most 
probable that it is by such a road that by far the largest part of 
the fauna of fresh water has left the sea. There are numbers 
of cases recorded of marine animals found in the course of 
rivers many miles from their mouth and beyond the reach of 
salt water, such as the Molluscan genus Scaphula, found in the 
Ganges 1,600 kilometres from its mouth. It is remarkable 
that the greatest number of such cases of intrusion are found 
1. See Trans. Norfolk and Norwich Nat. Soc. VIII., 1907, p. 410. 
