KEIGN OF THE ACEOGENS. 727 



Brongniart, from the investigation of the several geological for- 

 mations, has arrived at the conclusion that three distinct periods of 

 vegetation can be established. In the most ancient periods there is a 

 predominance of acrogenous cryptogamous plants (Ferns, Lycopodiaeese, 

 Equisetacese, and their allies) ; later, the predominance of gymnosper- 

 mous dicotyledons (Cycadacese and Coniferse) ; and, in the last place, 

 the appearance and predominance of angiospermous plants, both 

 dicotyledons and monocotyledons. These differences led Brongniart 

 to recognise three long periods of vegetable growth, which he terms 

 the reign of the Acrogens, the reign of the Gymnosperms, and the 

 reign of the Angiosperms, as indicative of the successive predominance 

 of each of these three great divisions of the vegetable kingdom, 

 rather than the complete exclusion of the others. Each of these 

 three kingdoms is commonly subdivided into many periods, during 

 which traces of the same family and genera are discoverable. These 

 periods comprehend many epochs, during which vegetation does not 

 appear to have undergone any notable changes. Materials are often 

 wanting to establish precisely these subdivisions, either from a want 

 of accuracy in determining the exact geological position of beds en- 

 closing vegetable impressions, or because the division of the various 

 species in the different beds of the same formation has not been care- 

 fully established. Brongniart proposes the following general division 

 of the fossil kingdom : — 



I. Eeign of the Aceogens. 



Cabbonifeeous and Pbrmian Periods. 



During these periods there seems to he a predominance of Ferns, a great 

 development of Lycopodiaeese, arhorescent forms of Lepidodendron and Sigillaria, 

 Gymnosperms allied to Araucaria, and anomalous Gymnosperms, as Noggerathia. 



II. Eeign of the Gymnospeems. 



Tbiassio and Jueassio Periods. 



Here we meet with numerous Coniferse and Cycadaceffi, while Ferns are less 

 ahundant. 



III. Eeign of the Angiospeems. 



Crbtaceods and Tertiary Periods. 



This is characterised hy the predominance of Angiospermous Dicotyledons, a 

 class of plants which constitute more than three-fourths of the present vegetable 

 productions of the globe, and which appear to have acquired a predominance from 

 the commencement of the Tertiary formations. These plants appear sparingly 

 even at the beginning of the chalk formation in Europe, but are more abundant in 

 this formation, as developed in North America. 



Schimper divides Brongniart's Eeign of Acrogens into two : — 1. 

 The Reign of Thallassophytes or of Cellular Cryptogams, especially 



