72 



CANADIAN FOSSILS. 



In the lower part of the Gaspd series representing the Lower Devonian, 

 the extreme abundance oi Psilophi/ton and Arthrodigma, and the occur- 

 rence of Prototaxites as the only representatives of the Conifers, give an 

 aspect of great antiquity to the flora. In the middle part of the same 

 series we find these forms associated with Lepidodendron and Stigmaricf 

 and Lepidofloios ; and at a corresponding horizon in New York and New 

 Brunswick we find Ferns, Cordaites and other types not occurring, a» 

 far as known, in the lower beds. I shall present in the sequel some rea- 

 sons for the belief that the middle of the Devonian is the true meeting- 

 place of the last survivors of an old Silurian flora and the earlier repre- 

 sentatives of the flora of the later half of the Palaeozoic. In the meantime 

 I may refer for a moment to views of the sequence of Palaeozoic plants 

 which might be entertained in accordance with theories of derivation of 

 species now prevalent. The ''^■ower Devonian is distinguished by the abun- 

 dance of some remarkable foru eferred to Algae of the genera Spirophy- 

 ton and Dictyophyton of Hall, uilo for the occurrence of vast quantities of 

 humbly organized acrogens suited for a semi-aquatic habitat, as Piilophy- 

 ton and Annularia. May not these two groups of plants be related in 

 the way of derivation ? Again, the synthetic types of acrogens of the 

 Lower Devonian, and the prototypal exogens of the genus Prototaxites 

 give way in the Middle Devonian to more perfect and specialized types of 

 acrogens and gymnosperns ; may they not have been advanced by a pro- 

 cess of evolution ? Such speculations have many charms for persons of 

 vivid imagination, and may be supported by the analogy between the pro- 

 gress of the development of the individual plant and the succession of 

 plants in geological time ; but the present case affords to them a sup- 

 port more apparent than real. The gap between Algse and acrogens like 

 Psilophyton with a well developed scalariform axis, is very great. The 

 Algae in question did not precede Psilophyton but were contemporan- 

 eous with it, and their association may be explained by the co-existence of 

 submerged shallows favourable to Algae, and swampy flats favourable to 

 Psilophyton and its allies, and by the alternation of these conditions in 

 the same locality. Prototaxites does not change into Dadoxylon. It 

 disappears and is replaced by a type of wood which continues to the pres- 

 ent day. Psilophyton continues to exist without improvement along with 

 the Lepidodendra and ferns of the Middle and Upper Devonian, and merely 

 becomes less abundant until it finally disappears. The phenomena are 

 rather those of the gradual extinction of an old flora and the introduction 

 of a new one from some different source. If therefore we desire to 

 account for the buccession of floras in this way, we must suppose local 

 extinction and the introduction from another region of plants which in the 





