GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 109 



and tertiary stages, would still be manifest, and the 

 several formations could be easily correlated. 



These observations, however, relate to the marine 

 inhabitants of the world: we have not sufficient data to 

 judge whether the productions of the land and of fresh 

 water at distant points change in the same parallel man- 

 ner. We may doubt whether they have thus changed: if 

 the Megatherium, Mylodon, Macrauchenia, and Toxodon 

 had been brought to Europe from La Plata, without any 

 information in regard to their geological position, no one 

 would have suspected that they had coexisted with sea- 

 shells all still living; but as these anomalous monsters 

 coexisted with the Mastodon and Horse, it might at 

 least have been inferred that they had lived during one 

 of the later tertiary stages. 



When the marine forms of life are spoken of as 

 having changed simultaneously throughout the world, it 

 must not be supposed that this expression relates to the 

 same year, or to the same century, or even that it has a 

 very strict geological sense; for if all the marine animals 

 now living in Europe, and all those that lived in Europe 

 during the pleistocene period (a very remote period as 

 measured by years, including the whole glacial epoch) 

 were compared with those now existing in South America 

 or in Australia, the most skilful naturalist would hardly 

 be able to say whether the present or the pleistocene 

 inhabitants of Europe resembled most closely those of 

 the southern hemisphere. So, again, several highly com- 

 petent observers maintain that the existing productions 

 of the United States are more closely related to those 

 which lived in Europe during certain late tertiary stages 

 than to the present inhabitants of Europe; and if this 



