398 THEORY OF PROGRESSION CHAP. XX. 



earth more and more highly organised types of animal and 

 vegetable life ; the modern species being, on the whole, more 

 specialised, i.e., having separate organs, or parts of the body, 

 to perform different functions, which, in the earlier periods 

 and in beings of simpler structure, were discharged in com 

 mon by a single part or organ. 



Professor Adolphe Brongniart, in an essay published in 

 1849, on the botanical classification and geological distribu 

 tion of the genera of fossil plants,* arrives at similar results 

 as to the progress of the vegetable world from the earliest 

 periods to the present. He does not pretend to trace an 

 exact historical series from the sea-weed to the fern, or from 

 the fern again to the conifers and cycads, and lastly, from those 

 families to the palms and oaks, but he, nevertheless, points 

 out that the cryptogamic forms, especially the acrogens, pre 

 dominate among the fossils of the primary formations, the 

 carboniferous especially, while the gymnosperms or coniferous 

 and cycadeous plants abound in all the strata, from the Trias 

 to the Wealden inclusive ; and lastly, the more highly deve 

 loped angiosperms, both monocotyledonous and dicotyledo 

 nous, do not become abundant until the tertiary period. It 

 is a remarkable fact, as he justly observes, that the exogens, 

 which comprise four-fifths of living plants, a division to which 

 all our native European trees, except the Conifers, belong, 

 and which embrace all the Composite, LeguminosaB, Um- 

 belliferas, Cruciferaa, Heaths, and so many other families, are 

 wholly unrepresented by any fossils hitherto discovered in the 

 primary and .secondary formations from the Silurian to the 

 oolitic inclusive. It is not till we arrive at the cretaceous 

 period that they begin to appear, sparingly at first, and only 

 playing a conspicuous part, together with the palms and other 

 endogens, in the tertiary epoch. 



* Tableau des Genres de Vegetaux fossiles, &c. Dictionnaire Universel 

 d'Histoire Naturelle. Paris, 1849. 



