168 F. E. Fritsch 
concentrated environment to withstand so radical a change (cf. 
also p. 178). 
Much of the evidence cited by Church as illustrating malnutrition 
in freshwater is open to a different interpretation. The relevant 
features of estuarine and salt-marsh vegetation, which he (p. 7) 
attributes to withdrawal from the open sea, may equally well be a 
result of the frequent changes in concentration of the medium 
owing to tidal influence. The statement that “all freshwater 
Plankton is on a lower plane in size, variety, and abundance” 
(footnote 4 on p. 7) is also open to question. There is no doubt a 
difference as regards size, but is that not more plainly related to the 
greater buoyant power of sea-water? As regards variety, however, 
it may be doubted if there is any appreciable difference, and under 
certain circumstances, as in warm waters (especially Tropics 1 ), the 
abundance of freshwater Plankton may quite well equal that 
customary in the sea. 
It does not appear therefore that there are any adequate grounds 
for the views {a) that conditions in freshwater are necessarily less 
favourable to growth than those in salt water, or (b) that freshwater 
Algae as a whole are reduced forms. It is however a striking fact 
that all Green Algae (excl. Siphonales) leave off abruptly at a level 
of morphological development which is far inferior to that exhibited 
by the true Seaweeds. It is at least a plausible assumption that 
Green Algae do not pass beyond this level, because all the more 
advanced forms have progressed landwards and given rise to the 
higher land-plants. They alone (likewise the Cyanophyceae) seem to 
have found more suitable conditions on the land and in freshwater 
than in the sea. The conclusion is that the transmigrant Green 
Alga was at about the level or perhaps slightly above the level of the 
present-day forms, at least in morphological development. The 
absence of more advanced forms in the series of the Green Algae is, 
on this basis, explained as due to failure to compete successfully in 
the sea with the Brown and Red forms and, for the rest, to their 
evolution into land-plants. 
It is particularly to be noticed that Green Algae (as also Cyano¬ 
phyceae and to a lesser extent Diatoms) show a very marked power 
of adaptation to life on land. In practically every series of Green 
Algae, as at present distinguished, there are terrestrial representa¬ 
tives, as illustrated by the following epitome: 
Tetraspor ales'. species of Palmella and Gloeocystis. 
1 Cf. Fritsch, Proc. Roy. Soc. London. Ser. B. 79 , 1907, p. 220. 
