STRATIGRAPHICAL SEQUENCES 231 



continuous ring of primary xyl&m 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 Sigillaria, 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 Arborescent 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." J 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 Trans. Roy. Soc.^ Edin., 1905, vol. xvi., p. 548. 



