416 THE FKESH- WATER LOCHS OF SCOTLAND 
menon, it struck me as very peculiar. In D. hyalina it was extremely 
striking to see this very same race in the course of only two to three 
weeks (May, June) everywhere changing into slender forms with pointed 
heads, the external appearance of which, moreover, was in every lake 
quite different. But still more remarkable was it to see how all these 
various summer races, when autumn came, slowly dropped their racial 
characters and in winter ended in the same clumsy, round-headed form 
common to all lakes. What means have we now to understand the 
sudden appearance of the very many summer races in spring, and to 
understand the appearance of the common winter race ? 
The occurrence of the numerous local races is favoured by the 
frequent 7nonogomc reproduction in plankton organisms (asexual 
formation of auxospores in Diatoms ; not constant and regular con- 
jugation, but mainly reproduction by partition in Cej^atium ; con- 
spicuous tendency to acycly in Rotlfera and Cladocera). Directions 
of variation once begun can therefore continue undisturbed ; no cross- 
ing from conjugation, and consequent disturbance and interruption in 
the directions of variation commenced, takes place. Resting-stages, 
resting-cysts, resting-eggs, etc., which as a rule are also the means of 
distribution of the species, are lost with the falling out of digonic re- 
production. In this way the races are separated ; each locality 
becomes an exclusive world to them ; they do not receive any impulses 
from without, and the racial characters can be preserved over great 
areas. 
The main causes of the disappearance of the sexual reproduction 
of the plankton organisms may be sought for in the fact that the pro- 
duction of the resting-stages, and especially their thick, chitinous 
skeletons, made so great claims on the mother-organisms that their 
organisation came into conflict with the demands made by the outer 
conditions ; i.e.^ in many localities the resting-stages made the mother- 
organisms too heavy. The result would be then, that the individuals 
forming resting-stages would sink down into deeper layers and perish. 
We are therefore able to understand the disappearance of resting- 
staeces through selection. 
How are we further able to understand that all the numerous 
summer races fall back into one and the same winter race ? To under- 
stand this phenomenon we must look beyond the boundaries of the 
small country in which these investigations have been carried out, and 
go back to other periods in the life-history of our globe. 
On comparing my own with the investigations of others in arctic 
alpine lakes, I was able to show in 1905, and later in 1906, that seasonal 
variations are restricted to the low-lying lakes of the temperate zone^ and 
are absent from the arctic, aljnne, and North European lakes in which 
the temperature did not for some time remain over about 12-16° C, 
