422 THE FRESH-WATER LOCHS OF SCOTLAND 
also occur. The evidence of such breaks in a chain of forms is not in 
the least in opposition to the view set forth here regarding the old 
collective species as chains of forms. 
Though I believe, as above stated, that these series of forms 
originate in the extreme north, this does not at all mean that the 
arctic forms with which the series begin owe their origin to the 
Glacial Age ; of this nothing is decidedly known, and 1 consider this 
opinion quite erroneous. 
Li recent years different scientists have tried to explain many 
facts connected with remarkable distribution or peculiarities relating 
to the biology of the plankton organisms as caused by the Ice Age. 
Some authors have advanced the idea that the whole fresh-water 
plankton should be regarded as a society which has immigrated into 
the fresh-water lakes from the Arctic Sea during the Ice Age. In my 
opinion, this idea is quite erroneous. The home of the fresh-water 
plcmldon must mainly he sought for in the Utloral and botto7?i regions 
of the laK'es^ and most of the fresh- water plankton organisms may be 
designated as bottom and littoral forms which have adapted them- 
selves more or less to pelagic life and made themselves independent of 
bottom and bank, where the great majority still pass a shorter or 
longer period of their lives. Reasons for this view I have set down in 
chap. xiii. of my Planlx'ton Investigations. With regard to the 
influence of the Glacial Age on the fresh-water plankton I share the 
view that we have been too hasty in making the Glacial Age 
responsible for the occurrence of forms at localities outside the true 
centre of distribution of the species, and for remarkable biological 
facts relating to the biology of the plankton organisms. Without 
going into details, I shall here merely summarise my opinions as 
follows. In the life-history of the fresh-water plankton the Glacial 
Age has only been a phase of transient importance : it has influenced 
the history of this community as well as of all other communities, but 
less than most. The individuals of the species which lived within the 
territories overtaken during the Glacial Period have suffered from this, 
but not the other individuals. This period lasted long enough to leave 
its mark on some characteristics of the species influenced by it. It 
affected their nutrition, their reproduction, and their shape. At the 
end of the Glacial Period races of these very old species, even then 
distributed over the whole earth, occurred in the northern part of the 
temperate zone, and owing to the Glacial Period they had become 
adapted to arctic or subarctic climatic conditions. The special 
characters impressed upon them by the Ice Period still persist, as is 
shown by the present races returning in winter upon the old races 
of this period (the present arctic races) and by their predilection for 
low temperatures. 
