STRATIGRAPHICAL SEQUENCES 231 
‘continuous ring of primary xylem is the older type of Sigillarian stem 
structure, and that the circle of isolated strands which form the primary 
xylem of the Clathrarian Sigillariae of the higher geological horizons has 
originated by a splitting up of the continuous-ring type of bundle; and 
as already mentioned, even in the few Clathrarian Sigillariae from the 
higher horizon of which the structure is known, the actual transition from 
the one type to the other can be observed.” 
“The Lepidodendra form, however, an older genus than Stigd//aria, and 
extend to the base of the Carboniferous Formation. In beds not far above 
the base, and low down in the Calciferous Sandstone Series, specimens of 
Lepidodendron showing structure have been found; and of two of these 
occurring in the same bed, one species shows the continuous ring of 
primary wood, while the other possesses a solid cylinder of primary wood 
without any trace of pith; and although there occur here the two types 
of primary wood, side by side, still the solid cylinder type seems to be 
more common in the lower than in the upper horizons of Carboniferous 
Rocks, and the sequence of changes in the development of the primary 
-xylem of the palaeozoic Arboreséent Lycopods seems to point to the solid 
vascular cylinder as the oldest type, from which has been derived the 
medullate cylinder with a continuous ring of primary wood, and this con- 
tinuous ring of primary wood has, in turn, broken up to form the isolated 
strands of primary wood found in the Clathrarian Sigillariae.”1 This is 
a good example of an evolutionary story, shown ‘among plants of near 
affinity in respect of a single character, and based upon stratigraphical as 
well as structural comparison. Similar conclusions are emerging at various 
other. points. 
Another result of importance derived from Palaeontological study is 
less direct in its bearings on the story of descent: it is that by comparison 
‘of fossils with modern plants certain stereotyped views, based primarily 
on the study of modern plants, are liable to be revised, and relaxed. 
This may be illustrated by reference to secondary thickening in stems. 
It was formerly held that stems which showed well-developed secondary 
wood were necessarily referable to Seed-bearing Plants. Difficulties followed 
from the acceptance of this doctrine, and they culminated in the case of 
the Calamarieae. Here the better knowledge of their anatomy, and of 
their fructifications showed clearly that a true Pteridophyte might attain 
large dimensions, and show a secondary thickening of its stem. Similar 
results are now familiar for other phyla of the Pteridophytes, and these 
facts, together with a better knowledge of recent plants, has shown that 
secondary thickening is a feature restricted to no single group of plants. 
Similarly, fossils have led to a relaxing of ideas respecting heterospory, 
and the seed-habit, and have helped quite as much as any study of recent 
forms, to the acceptance of a doctrine of parallel origin of marked char- 
acters independently in more than one line of descent. 
1 Zyans. Roy. Soc., Edin., 1905, vol. xvi., p. 548. 
