424 THE FRESH-WATER LOCHS OF SCOTLAND 
research I believe I have had my eyes opened to the influence which 
outer conditions may directly have on the transformation of the 
organism ; a somewhat secondary part is indeed also played by 
selection. 
We may return now to the question— How are we to consider all 
these single links in the chains of forms, all these local variations ? 
Do they belong to one and the same species, or may they simply be 
regarded as " Standortsmodificationen which, as soon as the in- 
dividuals are taken back to the localities where they originally lived, 
return to the original form of the local race from which they sprung ? 
In other words, are they comparable with the form series of Celebes 
snails described by the brothers Sarassin, or have we to do with 
chains of forms of geographically separated species fixed by inherit- 
ance, corresponding to those demonstrated by Wettstein in Euphrasia 
(1896) and Gentiana (1896, p. 307 ; 1900, p. 305), and after him by 
Sterneck in Alectorolophus (1901, p. 1)? 
On this matter we can at present form no definite judgment. 
It is for the rest obvious that researches on the formation of species 
in plankton organisms, as they penetrate more deeply into diverse 
domains, will yield quite different results. What is wanted here is 
experiment : until we have such before us each one may retain his 
own subjective opinion. 
As for me, I believe that a great part of the local variations are to 
be regarded as in fact constant forms {petits especes of the botanist), 
which may, however, be connected with forms not constant, but which 
under other conditions either return to or develop into one another. I 
therefore bring all the forms under single large collective species : 
D. long'ispina^ B. coregoni^ etc. If it should be objected that this 
standpoint is not quite consistent, I would answer that it is the 
nearest approach to nature, where all is in a fluctuating condition, in 
constant process of development. The sharp boundaries are man's 
work : any greater precision in description than the subject permits 
oversteps the aim, and should, in my opinion, be avoided. This in- 
vestigation has therefore tended more to delete the boundaries between 
forms than to make them more fixed. 
Su7mnary. — We have now endeavoured to give a brief review of 
the investigations of recent years on the variation of plankton 
organisms. That this variation is advantageous for the plankton 
organisms, and one of the means by which the cosmopolitanism of 
the fresh-water conmmnity is rendered possible, is in my opinion very 
probable. 
Owing to their form-changing power, the organisms are able to 
adapt themselves to the variations in the rate of sinking, and conse- 
