482 
president’s address. 
genus Pseudocuma has 13 species in the Caspian and only two 
elsewhere. It seems that, of the Sarmatian or Pontic genera 
surviving in the Caspian, many have found the changing con- 
ditions a stimulus to variability. This variability is also seen 
among the Cladocera of the plankton. These comprise a great 
variety of species of Polyphemidae belonging to genera which 
are either peculiar to the Caspian or have very few species in 
other seas. The genus Evadne is here represented by five 
species and numerous varieties, which seem to indicate that 
evolution is still proceeding. In addition to the genus Evadne, 
there are also two genera (Cercopagis and Apagis), which are 
very closely allied to the fresh water genus Bythotrephes, and 
one species of the genus Polyphemus. The genus Evadne, 
though probably itself derived from fresh water ancestors, is 
here a marine relict, while the other Polyphemids have, no 
doubt, been carried into the sea by rivers, and have there 
evolved from their parent form Bythotrephes. 
The Caspian Sea, unlike the Baltic, has therefore been a 
centre for the adaptation and evolution of a large number of 
animals characteristic of the open sea. But though it is of the 
greatest interest on this account, it can scarcely be fairly 
quoted as an example of the transformation of a marine into a 
fresh water fauna, as, from its size and its salinity, it still 
retains most of the characters of a true sea as opposed to a 
lake. 
One other lake should be mentioned as showing the incon- 
clusive nature of zoological evidence taken alone. This is 
Lake Baikal, which remains still one of the great unsolved 
zoological riddles. Its fauna comprises an extraordinary 
proportion of endemic species (90 per cent, in the case of the 
Mollusca), and a large number that are obviously of marine 
type. A seal (Phoca baicalensis) occurs in large numbers in 
the lake, and is doubtfully distinct specifically from the seal of 
northern Europe (P. annellata). The fish Comephorus is 
related to the Scomberidse, and a Sponge, Lubomirskia baica- 
lensis, is not only of marine type, but is even found living on 
the shores of Bering Island. On the other hand, the Amphipod 
