480 
PRESIDENT’S address. 
species such as Chiridotea entomon, which still lives in the sea 
itself, must show a very recent entry into fresh water, it is 
treading on very uncertain ground to draw conclusions from 
the occurrence of such genera as Limnicvthere, which, while 
belonging to a group of Ostracods characteristically marine, 
probably has occupied fresh water for a very long time. 
In the second place, the presence of a marine species in a 
lake is not necessarily evidence of its having acclimatised itself 
in that place. It is always possible, and generally probable, 
that it has entered the lake after adaptation to its new 
surroundings. 
But though the evidence from the so-called Relict fauna 
must be accepted with caution, there is no doubt that the 
fauna of fresh water has, in comparatively recent times, been 
enriched by the isolation of marine species in closed arms of 
the sea, but such additions to the fauna have been surprisingly 
few. 
It might be supposed that in the isolation of an arm of the 
sea, and the gradual freshening of its waters, an experiment in 
acclimatisation would be performed on so grand and prolonged 
a scale that a very large proportion of the fauna would become 
adapted to the changed circumstances. And yet, when we can 
follow such changes in all their stages, we find the result to be 
extremely small. In the Baltic Sea the changes can be 
followed in detail since Glacial times. At the outset the sea 
was broadly open to the North Sea, and contained a marine 
arctic fauna (Yoldia Sea). Then ensued a period of elevation 
and freshening, when the marine fauna gave place to a fresh 
water fauna (Ancylus Lake). Again an opening was found to 
the North Sea, and the Baltic became brackish (Litorina Sea), 
and the present conditions were approached. In each case we 
find with changed conditions a changed fauna, and the old 
fauna failing to make good its grouud. The Oyster, for 
example, though the adult can be acclimatised to live in fresh 
water, is strictly limited in nature within a certain range of 
salinity, and has disappeared in historic times from parts of 
the Baltic in which it was once abundant. It is true that in 
