226 Successsive Geological Dex>elopment. 



sea, and inclosing the memorials of the floras washed down 

 from the adjoining mountains. 



The mutual dependence of plants and animals on each 

 other is such, that we may fairly presume that the relative 

 numbers in each of these kingdoms of Nature did not depart 

 very widely in any former geological period, especially the 

 tertiary, from the proportion now prevailing. It is true that 

 the fossil flora of the palseontologist is meagre in the ex- 

 treme when contrasted with the fossil fauna, but this arises 

 from the fact that almost all our fossil species of animals are 

 aquatic, whereas there are comparatively few aquatic plants 

 now existing in the world, and they seem not to have been 

 more abundant at remote epochs. If we compare the ter- 

 restrial fossil fauna with the terrestrial flora, the dispropor- 

 tion no longer holds good. 



Professor Bronn has enumerated in his " Index Palseonto- 

 logicus," 24,000 fossil animals and only 2050 fossil plants, the 

 proportion being only one plant for twelve animals, whereas 

 in the living creation he estimates the relative proportions 

 to be seven species of plants for ten of animals. But al- 

 though the value of the botanical data on which we reason, 

 may only be as one to twelve when compared to our 

 zoological information, we seem already to have sufficient 

 evidence, that there have been at least four, if not five, revo- 

 lutions since the cretaceous era in the species of plants in- 

 habiting the earth ; and during these successive changes 

 there is no manifest elevation in the grade of organizlition, 

 implying a progressive improvement in the floras which suc- 

 ceeded each other from the eocene to our own epoch. The 

 plants of the lower eocene found at Sheppey include genera 

 of which the organization is as perfect as in those found much 

 higher in the eocene series (in the gypsum, for example, of 

 Paris) ; and the same may be said of the miocene and plio- 

 cene assemblages of plants. 



