88 M. Bronn on the Laws of Evolution of the Organic Worla 



they can only be regarded as exceptions to the rule. It is never- 

 theless true that if, in these considerations, we choose to descend 

 to the families of least importance, we shall see these exceptions 

 become multiplied. But although the laws which we have 

 just enumerated have, without the least doubt, presided over 

 the creation, we are far from pretending that they are as 

 mathematically absolute (excepting, of course, the negative and 

 decisive effects of the law of external conditions of existence) as 

 the law of universal attraction, the law of affinity, or any other 

 law which admits of no exception. Moreover we do not yet 

 know what rule the Creator himself has adopted for the deter- 

 mination of the systematic order of creatures. 



17. A great number of phenomena certainly appear to fulfil 

 the law of the successive development of series of organisms 

 answering to embryonal types, such as has been formulated by 

 M. Agassiz. Nevertheless the different characters presented by 

 organisms resulting from the metamorphosis of an embryonal 

 type are not all signs of a gradual perfecting. They are varia- 

 tions tipon a single theme of organization, upon a single funda- 

 mental idea. 



18. All the phenomena which we deduce from the law of 

 adaptation to the external circumstances of existence, from the 

 law of terripetal evolution, and from that of progressive develop- 

 ment, show us a regular progress from the commencement to 

 the close of geological epochs. Nevertheless there are two 

 moments which, from their importance, stand out by themselves 

 upon this uniform course of the history of the earth — one ter- 

 minating the palaeolithic, and the other immediately preceding 

 the caenolithic period. The former corresponds with the extinction 

 of the marshes of Stigmaria : this extinction involved the cessa- 

 tion of peculiar and very general phenomena on the surface of 

 the earth, which were intimately connected with the existence 

 of these singular bogs ; it also involved the disappearance of a 

 great number of palaeolithic types. The second corresponds 

 with the disappearance of Ammonites and Belcmnites, — the first 

 appearance, at all events on a considerable scale, of angiocarpous 

 Dicotyledons, of the Tcleostian fishes, of Birds inhabiting trees, 

 and lastly of Mammalia. By this means, the multiplication of 

 the number of genera and species received a fresh impulse. 

 From this moment date the first traces of a differentiation of 

 climates corresponding with the different zones of the terrestrial 

 globe. 



mammals are found so low down as in the Trias, the co-existenee of these 

 forms, until the extinction of the Dinosaurs in the Cretaceous era, would 

 appear to have extended over a long period of time, — contrary to the opinion 

 expressed by the author. — Ed. Annals. 



