2 PAL^ONTOLOGICAL BOTANY. 



has been the means of bringing geology to its present state of 

 advancement. The study of these extinct fonns has afforded 

 valuable indications as to the physical state of the earth, and 

 as to its climate at different epochs. This study requires 

 the conjunct labours of the Zoologist, the Botanist, and the 

 Petralogist. 



The vegetation of the globe, during the different stages of 

 its formation, has undergone very evident changes. At the 

 same time there is no reason to doubt that the plants 

 may all be referred to the great classes distinguished at 

 the present day — namely, Thallogens, including such plants 

 as Lichens, Alga3, and Fungi ; Acrogens, such as Ferns and 

 Lycopods; Gymnosperms, such as Cone-bearing plants and 

 Cycads ; Endogens, such as Palms, Lilies, and Grasses ; and 

 Exogens, such as the common trees of Britain (excluding the 

 Fir), and the great mass of ordinary flowering plants. The 

 relative proportion of these classes, however, has been different, 

 and the predominance of certain forms has given a character 

 to the vegetation of different epochs. The farther we recede 

 in geological history from the present day, the greater is the 

 difference between the fossil plants and those which now 

 occupy the surface. At the time when the coal-beds were 

 formed, the plants covering the earth belonged to genera 

 and species not existing at the present day. As we ascend 

 higher, the similarity between the ancient and the modern 

 flora increases, and in the latest stratified rocks we have in 

 certain instances an identity in sj^ecies and a considerable 

 number of existing genera. At early epochs the flora appears 

 to have been uniform, to have presented less diversity of 

 forms than at present, and to have been similar in the dif- 

 ferent quarters of the globe. The vegetation also indicates 

 that the nature of the climate was different from that which 

 characterises the countries in which these early fossil plants 

 are now found. 



