426 THE FEE8H-WATER LOCHS OF SCOTLAND 
I am inclined to believe that the plankton organisms, under the 
continually increasing demands of outer conditions upon them from 
north to south to diminish the rate of sinking, have down to a certain 
zone, which may perhaps be placed in the Mediterranean area, mainly 
responded to these demands by an increase in the cross-section resist- 
ance. As the species daring this process of adaptation approached 
too near to their limits of variation and the demands for diminishing 
the rate of sinking still continued, they took to the method of increase 
in superficial area through decrease in volume ; in this way possibly 
some racial characters disappeared. If this should prove correct, 
an interesting difference appears between the tropical fresh- water 
plankton and the tropical marine plankton, which is characterised 
by its immense formation of spines, processes, etc. The former has 
met the demand for diminishing the rate of sinking by increase in 
superficial area through decrease in volume ; the latter, by an increase 
in cross-section resistance by means of an extensive formation of spines, 
processes, skin-folds, gelatinous membranes, etc. 
In all structures tending to increase the cross-section resistance, 
i.e. the temporal variations, I am inclined to see the means by which 
the organisms try to answer the annual variations in the supporting 
power of the fresh water in a given locality, and in the increase in 
superficial area from diminution of volume the means by which the 
organisms during the distribution in the direction from north to south 
try to counteract the increasing rate of sinking in the same direction. 
It would be in full accordance with this theory if further investi- 
gations should prove, on the one hand, that the seasonal variations 
are only small in the tropics, because there, as well as in the arctic 
regions, the annual range of temperature in the plankton region of 
larger lakes is relatively slight ; and, on the other hand, that the 
superficial area, through diminution of volume and rich development 
of asperities, is greatest, because the supporting power of fresh water, 
though rather constant throughout the whole year, is less than in any 
other part of the globe. 
These suppositions are only put forward as working theories for 
future explorers of the tropical fresh-water lakes. 
PART in.— MAIN PROBLEMS OF FUTURE LIMNOLOGICAL 
INVESTIGATIONS 
Sir John Murray has further done me the honour to ask me to indi- 
cate here what kind of work I consider most needed at present in the 
science of limnology. I wish to call special attention to two points. 
As alreadv mentioned, we lack almost all knowledge of the tropical 
