ORGANIC NATURE. 275 



of different geological periods towards the plants and animals 

 which are living at the present day. On examining organisms 

 belonging to the earlier ages of the earth's history, forms are 

 found peculiar to those times, and foreign to the existing crea- 

 tion. But peculiar and strange as many of the ancient types 

 appear to us, they all possess manifold relations to existing 

 forms. They are constructed upon the same general plan. 

 Hence the circle of living nature embraces the plants and ani- 

 mals of former ages. Even the most ancient plants and animals 

 may find their place in systems of Natural History which are 

 founded upon existing forms ; nay, some genera of the present 

 time actually range back to the earliest geological periods. We 

 have already mentioned a genus of bivalves (Lingula, p. 233), 

 which makes its appearance as early as the Cambrian stage of 

 the Palaeozoic strata ; and in the Carboniferous formation and 

 the Lias Prof. Heer is acquainted with a whole series of genera 

 which agree with genera of the present day. As meteorites are 

 in a manner messengers from distant spheres, which tell us that 

 bodies moving in space consist of the same materials as the 

 earth, so these genera of fossil animals are messengers from the 

 most ancient times, declaring that from remote antiquity the 

 same laws have prevailed, and the same types have been mani- 

 fested down to our own time. 



The number of genera common to recent and to preceding 

 periods diminishes the higher we go in the series of animated 

 nature : common species first make their appearance in the 

 Cretaceous and Eocene periods. They are then few in number, 

 and confined to the lowest forms ; but they become more nu- 

 merous in the Middle Tertiary period, in which the genera were 

 already for the most part identical with those now in existence. 

 This approximation to living nature at the present day by no 

 means occurs equally in all classes. It is manifested earlier in 

 the simpler and older types of animals than in more recent and 

 more highly organized forms. In the very ancient class of the 

 Rhizopods some species identical with recent ones occur in the 

 Cretaceous strata, according to Ehrenberg ; but this does not 

 happen with the greater number of the species. Recent Mollusca 

 cannot be traced beyond the Tertiary epoch ; and Mammalia are 

 not only represented by different species in Tertiary times, but 



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