HOOKER LECTURE, 1922. 237 
past. But the goal of investigation must still remain the recognition of 
those great changes, in comparison With which the changes in the organic 
world would only appear as phenomena of the second order.” It is generally 
agreed that the Leptosporangiate Ferns became clearly defined as a class, and 
assumed an important rôle in the drama of plant-life subsequent to the 
Palæozoic era. 
Ferns existed in the forests of the Carboniferous-Permian epoch, but their 
precise relationship to existing types is not clear: the opinion may be 
hazarded, rash though it is, that the selected representatives of Mesozoic 
Ferns mentioned to-day are not directly connected by descent with Palæozoic 
ancestors. 
Between the shales and seams of coal rich in the remains of the luxuriant 
vegetation of the late Paleeozoic forests and the sun-cracked mud and barren 
sandstones of the Triassic period on which some of the early Dinosaurs left 
their footprints there is a difference which marks the parting of the ways. 
In the Eastern region of North America on the site of the yet unborn 
Appalachian mountains, as in other parts of the world, a period of prolonged 
sedimentation in Paleozoic ages was followed by crustal foldings and retreat 
of the seas: the trough of an ocean filled with thousands of feet of sedi- 
mentary material was converted into a lofty range of mountains. This 
“ Appalachian revolution,” as it has been called by American geologists, 
was “one of the most critical periods in the history of the earth. It is the 
physical expression of the beginning of a fresh cycle in the inorganic as in 
the organic world. In England the hummocks of Archean rock in Charn- 
wood Forest polished by wind-blown sand of Triassic deserts enable us to 
picture the arid wastes and salt-covered beaches of inland seas, to recon- 
struct the scenery familiar to the “clumsy reptiles " of the early Mesozoic 
age in both the Old and New Worlds. In Virginia, in the district of Lunz 
in Austria, and in some other parts of Europe sediments rich in plants 
demonstrate the occasional prevalence of conditions favourable to vegetation 
during the latter part of the Triassic period, but for the most part the climatic 
conditions of the continents would seem to have been in striking contrast to 
those associated with the very luxuriant development of the Permo-Carboni- 
ferous vegetation. The revolution in the earth’s crust with the concomitant 
interchange of land and water must have had its reflex in the organic world: 
land-connexions were made and destroyed, the factors governing climate 
were reshuffled, and the balance of life seriously disturbed, It is not merely 
missing chapters that give the impression of discontinuity in the history of 
life. The apparently sudden change in the general facies of the plant-world 
is the expression of an almost catastrophic adjustment to a new condition of 
stability in the crust of the earth. 
As new lands emerged from the sea, new lines of evolution may have been 
inaugurated ; the transmigration of marine plants which Dr, Church 
