LIMNOLOGICAL PROBLEMS 
425 
quently to shorten the resting-periods more than normally. This causes, 
as a rule, an increase in the number of individuals in the locality con- 
cerned, and is thus favourable to the species. It may be supposed that 
the supporting power of fresh water, as mentioned above, is different 
in different latitudes, and the possibility is not excluded that it 
diminishes from north to south. I am inclined to believe this, because 
the variability of the species, as mentioned above, is not equally great 
in the arctic and high alpine lakes and is slight in the North Euro- 
pean. On the other hand, it is greatest in the Baltic lakes, i.e. the 
variability of the species is smallest where the variations in the 
supporting power are smallest, and greatest where these are greatest. 
All these variations tend to augment the cross-section resistance, and 
thus to diminish the rate of sinking of the organisms. We may there- 
fore say that the cross-section resistance of the organisms increases 
from north to south. 
Further, it has been shown with regard to many of the most well- 
marked plankton organisms, that a decrease in volume takes place in 
the direction from north to south ; it therefore seems as if the organ- 
isms also undergo a relative increase in superficial area consequent 
upon a decrease in volume. Both phenomena agree very well with the 
fact that both the increase in cross-section resistance and the increase 
in superficial area owing to diminution in volume are in each locality 
most prominent in summer. Local as well as seasonal variations tend 
mainly to increase the form-resistance on the rise of temperature, viz. 
on the rate of sinking probably on the whole increasing. 
In order to disprove or confirm these views, knowledge of the 
appearance and mode of life of the plankton in tropical fresh-water 
lakes is necessary. We know nothing of the reproduction there, nor 
of the variations, either local or seasonal. The little to be gathered 
from the literature seems to suggest that adaptations to the extremely 
high summer temperatures, in so far as they come under the concep- 
tion of change in shape, consist less in an increase of the cross-section 
resistance through extensive formation of floating apparatus, and more 
in an increase of the superficial area due to a decrease in volume. 
With this increase in superficial area there is in the tropical fresh- 
water plankton an apparently distinct tendency towards making the 
surface rough by means of numerous closely placed roughnesses, 
reticulations, etc. It is a remarkable fact that no organism has yet 
been observed in tropical fresh-water lakes which through extensive 
formation of floating apparatus is specially adapted to water with 
high temperatures and slight supporting powers : the Hyalodaphniae 
m Victoria Nyanza seem quite similar to our own. The most con- 
spicuous structure I know of is the very large mucrones in Bosminae 
from the river Amazon (Stingelin, 1904c, Tab. 20, f. 6). 
