18 William J. Hodgetts 
it shows some relation to the concentration of the water, while 
competition with other species (cf. Eudorina below) is probably a 
very important factor. 
With regard to the concentration of the water the most favourable 
conditions appear to be when the concentration is gradually rising 
between 16 and 20 degrees, since a pronounced rise in the frequency- 
curve of the species (e.g. in July 1918, May 1919, and July—August 
1920) always coincides with these concentration-conditions. In July 
and August 1919 concentrations above 20 degrees (cf. Fig. 3) were 
probably the cause of Pandorina becoming rare comparatively early 
in the year, although competition with Eudorina (or the attacks of 
aquatic animals) may also have had something to do with bringing 
about this result. 
Eudorina elegans (Fig. 10) tends to commence its development in 
May, at about the same time as Pandorina, but in contrast to the 
latter species it may persist in quantity for some time after the 
decrease of Pandorina in early autumn, and thus appears to be a 
rather more hardy form. A comparison of the curves of Pandorina 
and Eudorina (Fig. 10) is rather interesting, and shows that it seems 
impossible for these two algae to attain any considerable development 
in the pond at the same time 1 . In 1920, for example, the increase in 
Eudorina during May and June apparently caused Pandorina to 
remain rare, but when Eudorina decreased in July the Pandorina 
suddenly became “rather common” (in early August). Again during 
June—July 1919 a rise in the Eudorina- curve involved a fall in that 
of Pandorina ; and in August 1918 a fall in the Pandorina- curve 
apparently enabled Eudorina to develop. This appears to be evidence 
of the existence of mutual competition between these two algae, such 
competition, however, acting adversely upon one of them only when 
the other has attained a fair degree of development in the pond. 
The only other explanation appears to be that the two species are 
favoured by a different set of external conditions, and that the 
conditions which favour one species exclude the pronounced growth 
of the other. 
From a comparison of the various curves for 1919 and 1920 it 
might be deduced that Eudorina is favoured by a somewhat higher 
concentration of the water than Pandorina 2 , but the evidence in 
1918 points to exactly the opposite conclusion. Another point is that 
1 The same thing is apparent from the frequency-data of Pandorina and 
Eudorina given by Fritsch and Rich (18) in their study of Barton’s pool. 
2 A conclusion arrived at by Fritsch and Rich (18) in the case of these two 
species in Barton’s pool. 
