Thalassiophyta and Algal Ancestry of Higher Plants 173 
to which the transmigrants were subjected, there can be little 
difference of opinion except as regards illumination and the moisture- 
content of the air. On these points Church himself is not clear, since 
on p. 20 he speaks of “a subsaturated atmosphere and a sky of fog 
and cloud” and on p. 21 of “the brilliance of open sunlight.” It is 
difficult to conceive of any possibility of transmigration, whether by 
filamentous or more massive forms, unless the air was almost 
saturated and there was copious rainfall during the greater part of 
the period. Even if that was so, however, there must have been an 
appreciable increase in light-intensity for an organism emerging from 
the water, and it is difficult to follow Church when he suggests (p. 34) 
that " types with the elaborated factors of such shade-flora (Florideae) 
will have the better chance of survival under conditions of malnu¬ 
trition.” All recent experimental work goes to show that shade-loving 
Algae are extremely sensitive to any increase in light intensity. 
The most serious problems for the transmigrant must have been 
water-supply and the maintenance of an erect habit, since it may 
be doubted (cf. above) whether the change in the nutritive value of 
the solution was at all as profound as Church supposes 3 . The abundant 
terrestrial Algae belonging to the successful groups testify to the 
capacity of these to exist through a period of diminished water-supply 
and, as above suggested, this capacity may have been one factor 
that led to their success in transmigration. It may be granted that 
it is not easy to picture the origin of even the simplest land-plant 
from a filamentous Alga, but there is no more difficulty in it than 
in accepting Church’s view. The larger the transmigrant the more 
acute would be the problem of water-supply at first and it is difficult 
to conceive of the persistence of an erect Alga 3-6 feet long (Church, 
p. 90), gradually raised out of the water; and if the transmigrant 
was not erect from the first, it is not easy to comprehend how such 
a habit could be subsequently attained 2 . 
It may be that the land- and water-forms of present-day aquatics 
give us some indication of what the effect of the assumption of the 
terrestrial mode of life may have been, viz. a general condensation 
1 As regards this point, if freshwater Algae are derived from marine forms 
and have been able to survive the “malnutrition” involved in the change, 
why should not similar forms have survived and gradually developed into 
higher land-plants ? 
2 Church (p. 21) cites an observation of Migula’s on Nitella mucronata 
growing erect to a height of io inches in subsaturated air “with no other 
mechanical assistance than the turgidity of its non-corticated axial cells,” but 
since such a plant has little weight to support it can scarcely be regarded as 
affording evidence for his hypothesis. 
