392 Professor Agassiz on the Development of Organised Beings 



from the differences observed between the different races of 

 domestic animals, cannot in any way weaken the general prin- 

 ciple of the fixity of species. For, to place in the same line, 

 phenomena so different as that of the succession of different 

 species, genera, families, and classes, and the partial and in- 

 constant modifications to which, under the influence of man, 

 certain animals which he has attached to himself, and certain 

 cultivated plants, have been subjected, is at once to declare 

 incompetency to discuss questions of this nature. 



But because the organized beings of these different ages of 

 nature have not a genetic bond of connection of the nature of 

 a successive sexual procreation, we must not hence conclude 

 that they are not members of one same plan, and that they 

 are not linked together by bonds of a more elevated descrip- 

 tion, as we shall afterwards find to be the case. 



The only real difficulty on this point which remains to be 

 solved, is the rigorous determination of the limits of all 

 these great epochs ; for in proportion as the investigation 

 of fossils acquires more precision, the number of these dis- 

 tinct epochs seems to increase. It is already ascertained 

 that the oldest formations, as far as, and including the coal 

 deposits, are characterised by a particular order of things. 

 In the more recent formations from the gres bigarres up to the 

 chalk, a second great epoch has been recognised, differ- 

 ing as much from the first as from the tertiary epoch that suc- 

 ceeded it, the latter terminating before the present creation , 

 to which belong man and his contemporaries. These four 

 great epochs, that may be called the ages of nature, are sub- 

 divided into distinct periods, which are equally characterised 

 by several peculiar features. 



If it were allowed me to enter into some more circumstan- 

 tial details, I would add, that those who believe that during 

 the first epoch there existed only animals of an inferior or- 

 ganization, are strangely deceived. Far from that, from the 

 earliest period, the four types of the animal kingdom have 

 been represented at the surface of the globe : the Radiata, 

 Mollusca, Articulata, and Vertebrata appeared simultaneously 

 as the first inhabitants of the earth ; and at each of the following 

 epochs, new types of these same great groups reappeared in 



