230 EVIDENCE FROM PALAEOPHYTOLOGY 



are clearly apprehended that the true value of that evidence will begin to 

 emerge. Though, as we see, it cannot yet be held to throw any direct 

 light on the prime origin of terrestrial plants, still it has valuable bearings 

 on the mutual relations of the earlier known types. It is especially valid 

 in supplying . a knowledge of " synthetic types," that is, plants now extinct, 

 which include among their characteristics some of the peculiarities of two 

 or more distinct lines of descent. The most important of these hitherto 

 disclosed are the Sphenophylls, which constitute a series separate from the 

 three great phyla of living Pteridophytes, though some affinity is to_be 

 recognised between them and the modern Psilotaceae. Their leaves agree 

 with those of the Equisetales in being whorled, and being superposed 

 they are most nearly like the oldest known Calamite Archaeocalamites. 

 Their whorled arrangement also corresponds with that of one of the 

 earliest Lycopods, Lycopodites Stockii, from the calciferous sandstone. 

 The anatomy of the stem of Sphenophyllum is Lycopodial rather than 

 Equisetal, but the strobili are nearer to those of the Equisetales than to those 

 of any other known family. The interest in the group which showed such 

 mixed characters was further intensified by the discovery of Cheirostr obits. x 

 " This strobilus presents the same combination of Lycopodial with Equi- 

 setal characters which we find in Sphenophyllum itself, but in both directions 

 the agreement is more striking. . . . We may express its probable natural 

 position by placing it in the main division Sphenophyllales, but in a 

 family by itself, distinct from the Sphenophylleae in the narrower sense. 

 The threefold affinities of Cheirostrobus, firstly with the Sphenophylleae, 

 secondly with the Equisetales, and thirdly with the Lycopodiales, appear 

 indisputable, and indicate that this genus, and the Sphenophyllales gene- 

 rally, represent a phylum intermediate between the other two, which we 

 must suppose to have originated with them, from a common ancestral 

 group. In this way, the study of the extinct Sphenophyllales has thrown 

 quite a new light on the obscure affinities of the Equisetal stock, for it 

 indicates clearly that this phylum had a common origin with that of the 

 Lycopodiales, a conclusion which the exclusive investigation of their recent 

 representatives could never have suggested." Another important synthetic 

 group of plants, of early occurrence, is 'that of the Cycadofilices, which 

 link together the Pteridophytes and the Gymnosperms. Such examples 

 illustrate what may be held to be the most important results obtained 

 hitherto from Palaeophytology, as aiding the study of descent in Plants. 



Another line of argument from Palaeontological data is now beginning 

 to ,be used, though only sparingly, since it is rare as yet to find that the 

 facts suffice for its application. It consists in the comparison of plants of 

 near affinity from different strata, and deducing from their stratigraphical 

 sequence a progression as regards some single character. This method 

 has been carried out successfully by Mr. Kidston, in respect of the structure 

 of the stele of Lycopods: he has concluded that "it is probable that the 

 * See Scott, Studies in Fossil Botany, pp. 494-497. 



