James Small. 
275 
A far more favourable condition for this transmigration is to 
be sought in the variable nature of the sea-level, due to crustal 
movement. We know that during the Paleozoic the oceans 
invaded North America at different times to the extent of millions 
of square miles. These overfloodings were shallow seas—epeiric 
seas—with an average depth of probably less than 500 feet. Now 
think of these seas withdrawing very slowly from the land, and see 
what a favorable ground for humus must have been left in their 
wake, where the seaweeds might become adapted to the new 
environment that was thus forced upon them. Just as the fishes 
had to develop legs and lungs if they were to become the land¬ 
living amphibia of the Silurian or Devonian, so the Thalassiophyta, 
probably as early as the Archeozoic, were forced to become 
Xerophyta. 
Yale University. 
PRELIMINARY NOTE ON A HYDRION DIFFERENTIATION 
THEORY OF HELIOTROPISM. 
By James Small, D.Sc. 
F URTHER work, which is now proceeding on the influence of 
carbon dioxide on geotropism and heliotropism, having 
indicated that the carbon dioxide balance is, indeed, of the 
fundamental importance previously suggested (1, p. 51 and 2, p. 208), 
a hydrion differentiation theory of heliotropism has been developed 
As it will be some time before the experimental work now in 
progress can be completed, the theory has been considered of 
sufficient interest in itself to warrant a brief description. 
It has been suggested, and there is more experimental evidence 
accumulating in support, that the direction of geotropic curvatures 
is governed by the hydrion concentration of the continuous phase 
of the plasma membranes of the perceptive cells. The present 
theory is that the same is true for heliotropic curvatures. 
Briefly, it is suggested that light, either by increasing photo¬ 
synthesis and the amount of respirable material or more probably 
