414 THE FRESH-WATER LOCHS OF SCOTLAND 
animal for adapting itself to the variations in outer conditions, are 
dependent on the intensity with which the moults proceed. In this 
the explanation might perhaps be sought as to the phenomenon 
which, of all those connected with seasonal variations, has always 
appeared to me the most puzzling. It is easily understood why the 
organisms in spring, when the rate of sinking rises, transform their 
bodies so that the floating power becomes greater ; but in all the 
earlier researches I did not see for a long time how to explain 
why these floating apparatus when once formed are again drawn in 
towards winter. Their formation in spring is necessary, and may well 
be thought of as brought about through selection, but I was unable 
to see any necessity for their being drawn in in autumn. If it were 
possible to show, however, that the body is only transformed during 
and soon after the moults, and that the frequency of the latter is 
dependent on outer conditions — first and foremost on temperature, so 
that they proceed most rapidly and frequently at high and increasing 
temperatures, more slowly and rarely at lower and decreasing — the 
explanation would also be found here of the phenomenon that the 
Cladocera are transformed in spring but in autumn cease transform- 
ing:. Thus the return to the common race would not occur because 
the organisms no more " needed floating apparatus and therefore 
drew them in, but for the very simple reason that the organisms when 
the moulting processes ceased were practically not able to transform 
themselves. 
The very peculiar fact that the seasonal variations do not proceed 
gradually but by a sudden change, which occurs in all plankton at the 
same period (May, June), when the temperature is at 12-14° C, and 
is completed in the course of two to three weeks, inakes us understand 
that the seasonal variations really may he of the greatest significance for 
the plankton organisms. 
2. Local Variation. — We are now able to understand how the 
seasonal variations take place, and how the plankton organisms of the 
fresh water are able, by means of variations in the body form, to follow 
the variations in the buoyancy power of the fresh water. 
The investigations on which my results have been based were not 
carried out in a single lake, but in many, so that I was able not only 
to follow the seasonal variations in many lakes, but also to study the 
local variation of the plankton organisms. 
These studies gave the following, very peculiar, main results. 
Although the seasonal variations, which have everywhere the same 
object, proceed on parallel lines in different localities, considerable 
differences may nevertheless assert themselves both with respect to the 
amount of variation in each locality and with respect to details in the 
manner in which the organism meets the demands for variation in 
