176 SECTIONAL ADDRESSES. 
newly assumed after transmigration, but that they merely represent 
an adaptation to sub-aerial conditions of a differentiation already attained 
at the phase of marine phytobenthon (rooted sea-weeds). At the same 
time it is not suggested that any existing class of sea-weeds can be taken 
as representing the ancestry of the Land Flora; the transmigrant races 
are, as Alg, extinct—they may have been Green Alge of a high grade 
of organisation, on a level now perhaps most nearly represented by the 
highest of the Brown Seaweeds. 
Thus the transmigrants, which were destined to become the parents 
of the Land Flora, are pictured as already highly organised and well- 
differentiated plants, which only needed to provide themselves with 
absorptive instead of merely anchoring roots, and with a water-con- 
ducting system (xylem and stomata) in order to fit themselves for sub- 
aerial life, while, on the reproductive side, the great change remaining 
to be accomplished was the adaptation of the spores to transport by 
air instead of by water. 
It is clearly impossible to criticise the theory in detail, for the 
assumed transmigrants are ex hypothesi unknown; we can only form 
a distant conception of what they were from the analogy of the highest 
sea-weeds of the present day, which admittedly belong to quite different 
lines of descent. Dr. Church puts the transmigration so far back 
(pre-Cambrian) that not much help can be expected from fossils, but 
to this subject we shall return. 
Some botanists find a difficulty in accepting the suggestion that 
plants already elaborately fitted out for a marine life could have sur- 
vived the transition, however gradual, to a totally different environment. 
Such thinkers prefer to believe that lower forms may have been 
more adaptable, and that morphological differentiation had, in a great 
degree, to start afresh when the land was first invaded. My own 
sympathies, I may say, are here with Dr. Church, for I have long 
inclined to the belief that the vascular plants were, in all probability, 
derived from the higher Thallophytes. The view of the late Professor 
Lignier, now so widely accepted, that the leaf, at least in the mega- 
phyllous or Fern-like Vascular Plants, was derived from specialised 
branch-systems of a thallus, assumes, at any rate, that the immediate 
ancestors possessed a well-developed thallus, such as is now known 
only among the higher Alge. The Hepaticse, as we now know them, 
clearly do not come into question, and the Pro-hepatics, which Lignier 
postulated as early ancestors, have only a theoretical existence, and if 
they were ever present in the flesh may well have been transmigrant 
Algee. 
The question now arises, how far have we any evidence from the 
rocks, which may bear on the transmigration and on the nature of the 
early Land Flora? A very few years ago no such evidence was avail- 
able—such data as we then possessed seemed too obscure to discuss. 
Quite recent discoveries, especially those from the famous Rhynie 
Chert-bed, have shown that in Early Devonian times certain remarkably 
simple land-plants existed, which in general configuration were no 
more advanced than some very ordinary sea-weeds of the present day. 
'At the same time these plants were obviously fitted for terrestrial life, 
