176 M.Bronn on the Laws of Evolution of the Organic World 



the ancient periods, not only to the threshold of the present 

 creation, but also into its very heart. We have seen that these 

 modifications do not by any means all cease at the threshold of 

 existing nature, but that tbey often continue their course with- 

 out interruption, so that it becomes difficult for us to say exactly 

 where this threshold is to be found. The groups of plants or 

 animals which were in course of rapid diminution at the close 

 of the geological ages have continued diminishing in the exist- 

 ing epoch : amongst all the fossil species the marine Mollusca are 

 those with which we are best acquainted, and the study of which 

 is most decisive upon this point. Those, on the contrary, which 

 were in course of augmentation have continued their develop- 

 ment. At first there existed a certain number of orders and 

 suborders entirely foreign to our existing creation ; and all the 

 genera, with the exception of from 1 to 3 per cent., were different 

 from those of the present day. By degrees the number of these 

 foreign types diminished, and the number of genera which have 

 persisted to our days became more and more considerable. In 

 the sequence of ages this number rose gradually from 20 to 40, 

 60, 80, 90, and, lastly, 100 per cent. By degrees, and this 

 even at the close of the Cretaceous period, some isolated species 

 made their appearance, which have persisted to the present day. 

 Starting from the Eocene period, their number rose gradually to 

 20, 60, 80, 90, 95, and 99 per cent., although it is not yet pos- 

 sible to demonstrate an equally regular gradation for all the 

 classes. But however gradual may have been this passage from 

 the geological faunas and floras to existing nature in the south 

 of Europe (according to Philippi), it is very possible that in 

 other countries a great portion of the series of intermediate 

 beds may be wanting, and that consequently in these regions 

 the distinction between the marine formations belonging to 

 these two periods may appear far more marked than in Eu- 

 rope. In the same way, the separation of two older forma- 

 tions may be far more distinctly marked in one country than in 

 another, for perfectly analogous reasons. This gradual passage 

 from the ancient creations to the existing one does not manifest 

 itself solely in the constantly increasing proportion of identical 

 species, but also in the constantly increasing differentiation of 

 the floras and faunas in accordance with zones, from the Eocene 

 period to the present day. The formation of local floras and 

 faunas, from the Eocene epoch to the Pliocene and Diluvian 

 epochs, exhibited the same local characters as the floras and 

 faunas of the present day : already each country was inhabited 

 by the same characteristic families, the same genera, and a great 

 part of the species which we find dwelling there at present. 

 The most recent tertiary strata of England contain a testaceous 



