420 THE FRESH-WATER LOCHS OF SCOTLAND 
series of forms are different. With regard to the plankton organisms, I 
believe that they are connected with the more varied and transitory 
changes in the surroundings caused by the melting of the ice and the 
subsequent improvement in the climatic conditions after the Ice Age. 
As all the changes in the shape of the plankton organisms tend to 
reduce the rate of sinking, all being mutually connected and parallel 
with the rising temperature and the decreasing viscosity, vertically 
through time as well as horizontally from north to south, I conclude 
that for the plankton organisms it is a conditio sine qua 7ion to follow 
the variations in the supporting power of the fresh water, which again 
are dependent upon temperature and concentration. As we now 
further know that the temperature, though with fluctuations, has 
risen from the Glacial Age to the present day, I would also con- 
clude that the rising temperature subsequent to the improvement in 
climate after the Glacial Age was the direct external stimulant 
responsible for the occurrence of these series of forms. 
As the difference between summer and winter temperatures and 
the consequent yearly variation in the supporting power of the water 
continually increased over more extensive areas, the species were con- 
stantly forced nearer to the limits of their range of variation on 
endeavouring to adapt themselves to the decreasing supporting power. 
The sexual periods in the pelagic colonies of the large lakes were at 
the same time more and more on the decline, and consequently local 
races arose. Through seasonal variations these races adapted them- 
selves to the buoyancy conditions of the locality, and through selection 
the single links during wanderings arranged themselves in series of 
forms from north to south. 
I have long held bhe view that the way in which variations in the 
outer conditions contribute to the occurrence of morphological series 
of variations is, that a biological separation has preceded the morpho- 
logical division. Behind the morphological variations are the biological; 
both are but rarely printed on paper or preserved in museums, but he 
who lives much in nature has them before his eyes every day. Outer 
conditions first influence the mode of life of the organisms ; the 
modifications in the latter through increased or decreased use of 
certain functions or structural characters then cause those differences 
to appear by means of which the diff'erent stages in the morphological 
chains can be distinguished. The division of the species brought 
about by variations in outer conditions often remains in the biological 
stage. Outer conditions separate a species into a number of groups 
of individuals differing biologically, but not to be distinguished 
morphologically. Examples of this must be sought for at present 
mainly among the lowest organisms, yeast and bacteria ; but there is 
no doubt that even if the number of such biological species known 
