K.—BOTANY. 2438 
co-ordinate with physiology, was, of course, the general acceptance by 
biologists cf the doctrine of descent with modification, popularly called 
evolution. Belief in the reality of this process at once invested the 
comparative study of structure with a new fascination. LHvery part, 
every organ of an animal or plant, could be interpreted in the light of 
the doctrine of descent. All the species of a group should, according 
to the theory of descent, be theoretically traceable to a hypothetical 
‘common ancestor’ of the group, and these group ancestors again to 
remoter ancestors. Ultimately we should be able, theoretically at 
least, to reconstruct the whole genealogical tree of the plant and animal 
kingdoms. It was, of course, recognised that we could never hope to 
complete this task, even if we possessed an exhaustive knowledge of the 
structure and development of every kind of organism now living, for 
very many forms had been destroyed and had disappeared altogether 
in the course of the evolution of the organic world as 16 exists to-day. 
But the remains of many of the organisms which, had lived in past ages 
were still preserved as fossils, and a knowledge of their structure would 
substantially help us on the way to the goal, even though that goal 
could never actually be reached. Though the geological record was 
extremely fragmentary, yet it did bring to our knowledge many kinds 
of plants, some more or less closely allied to living forms, others which 
could not be placed in any living group, and others, again, which sug- 
gested that they might represent or at least stand near to the common 
ancestors of existing groups. 
If we consider the most recent developments of the subject we find 
that, on the whole, the search for common ancestors as such has been 
disappointing. The ‘seed-bearing ferns’ (Pteridosperms) have turned 
out to be, so far as we can tell, a perfectly independent group having 
no demonstrable connection with the true ferns. The most primitive 
fossil ferns known (the Primofilices or Coenopterideze of the Lower 
Carboniferous) certainly represent a very ancient group. But not only, 
according to Dr. Scott in his most recent statement,* do ‘ Pteridosperms 
and Ferns at all times show themselves perfectly distinct’: ‘we are 
dealing, in the Lower Carboniferous Primofilices, with early races 
already specialised on their own lines, and probably only indirectly 
connected with the main current of Fern-evolution.’ 
The remarkable Rhynie fossils described by Kidston and Lang from 
the Lower Devonian—the oldest vascular plants with structure pre- 
served that are as yet known—have revealed in the genera Rhynia and 
Hornea a leafless and rootless type with large simple terminal sporangia 
and a simple stele occupying the centre of the axis. These plants show 
striking points of agreement with the living Psilotales, but their dis- 
coverers, so far from being prepared to assert that they are prototypes 
of Psilotales, create for their fossils a new class, the Psilophytales. 
Thus we have now recognised six distinct classes or orders of living and 
fossi! Pteridophytes,* and parallel with these six distinct classes of non- 
on D. H. Scott, ‘The Early History of the Land Flora,’ Nature, Nov. 11, 
_* Psilophytales, Psilotales, Sphenophytales, Equisetales, Lycopodiales 
Filicales. 
