Thalassiophyta and Algal Ancestry of Higher Plants 167 
phyceae and Cyanophyceae can equally well be regarded as having 
remained at a low horizon of development. 
It may moreover be pertinently asked in this connection why, 
if freshwater Algae are reduced, the marine Chlorophyceae (and 
Cyanophyceae)—in what Church considers to be the more favourable 
environment—do not exhibit any higher organisation than their 
freshwater allies. Church (p. 30) contrasts the freshwater Vaucheria 
with the marine Codium, the former being regarded as having a 
reduced thallus, but, apart from the absence of all evidence of 
reduction, there are a considerable number of marine Codiaceae 
[Aurainvillea, Udotea minima) in which the organisation of the 
thallus is not much in advance of a Vaucheria or Dichotomosiphon 1 . 
As far as I am aware too, there is nothing to choose between the 
marine and freshwater Phanerogams, either as regards dimensions 
or prolific growth. In short a comparison of the existing forms which 
grow in both types of habitats far from supporting the view advanced 
by Church seems to indicate that conditions of plant-growth in the 
sea are less favourable for green plants than in freshwater. 
Church points out (p. 7) that the chemical content of fresh as 
opposed to salt water is strikingly inferior, a fact about which there 
can be no difference of opinion, but this does not necessarily imply 
that nutrition is or was easier in salt than in freshwater. Too little 
is known about the nutrition of Seaweeds to enable one to express 
an opinion on the point 2 . Moreover it may be doubted if we can 
safely assume that the primeval sea was at all as rich in mineral 
salts as that at the present day; according to Schuchert 3 “the 
Archeozoic oceans had far less salts and probably a different salt 
combination.” The change from marine to freshwater conditions in 
those remote times may not have been anything like as pronounced 
a one as it would be at the present day and this may perhaps be the 
clue to the reason why transmigration has not occurred in more 
recent epochs, modern Seaweeds being too highly adapted to their 
1 Church (cf. loc. cit. pp. 34, 46) accepts West’s view that the Desmids, 
certainly among the more successful and ubiquitous Green Algae, are reduced 
from filamentous forms. It has however repeatedly been pointed out (Tansley, 
in New Phytol. 4 , 1905, p. 145; Fritsch, in New Phytol. 16 , 1919, p. 5) that all 
the evidence cited in favour of this view will read equally well the other way. 
Few algologists would follow Church (p. 8) in regarding Hydrurus as a reduced 
filamentous type. 
2 On the whole Green Algae are more accommodating as regards considerable 
variations in the concentration of the surrounding medium than the true 
Seaweeds (cf. for instance Oltmanns’ Morph, u. Biol. d. Algen, 2 , 1905, p. 178), 
3 New Phytol. 19 , 1920, p. 274. 
