LIMNOLOGICAL PROBLEMS 
397 
appearance of the lakes proceed more slowly in the alpine than in 
the Baltic lake-area, and are most probably due more to the material 
carried along by the rivers, which is mainly inorganic, than to the 
organic material produced in the lakes. It appears, however, from 
Bohm's records, that the alpine lakes also disappear, that in the Tirol 
118 alpine lakes have disappeared since 1774 ; and from Walser's, that 
of 149 lakes in North-East Switzerland 75 smaller ones have quite 
disappeared and 16 have been greatly reduced since 1668, in the course 
of about 250 years {vide Huber, 1905, p. 1 ; see also Friih and 
Schroter, 1904, p. 20). The influence of man on the filling up of the 
lakes and on the chemical quality of the water is probably hardly so 
great as in the Baltic lakes. 
It appears from the foregoing that the alpine lakes present much 
greater similarities with the North European and arctic lakes than 
with the lakes of the Central European lowland ; a detailed demon- 
stration of this is quite superfluous. A great many phenomena which 
we have been able to record from the alpine lakes may certainly also 
be pointed out in the North European and arctic lakes, but have not 
been dealt with here, as we do not know anything as to their trans- 
parency or colour. The agreement of the physical conditions in the 
high alpine lakes with those in the arctic lakes has sufficiently often 
been advanced, recently by Zschokke ; in most regards they also present 
quite the same appearance. Still, in one regard which has hitherto 
probably not been sufficiently emphasised they diff'er greatly, viz. in 
light. The long dark arctic winter night is not paralleled by anything 
in the high alpine lakes : the yearly quantity of light received by the 
alpine lakes is many times greater than that of the arctic region. In 
those cases where the high alpine lakes are covered with ice but free 
from snow, or where the snow does not cover the whole surface, this 
much greater quantity of light will, at any rate for the phytoplankton, 
be a principal factor in rendering active life possible where it is 
impossible in arctic regions. 
Tropical Lakes 
Omitting the Mediterranean lakes, since only future investigations 
can show whether these are to be regarded as a special type, we 
may briefly deal with the tropical lakes. 
We may first of all mention the enormous quantity of light which 
penetrates the surface of the water, and also that the amount of light 
as compared with that in higher latitudes is not subject to such great 
and regular annual variations as is the case in the temperate and 
especially the arctic regions. Further, it must be remembered that the 
annual range of temperature variation is probably relatively slight — 
