SUBDIVISIONS IN GEOLOGICAL HISTOllY. 401 



^poch has had its peculiar species, or j^eculiar groups of species. Moreover, 

 the succession of life has followed a grand, law of progress, involving under 

 a single system a closer and closer approximation in the species, as time 

 moved on, to those which now exist. It follows, therefore, that identity of 

 sjyecies of fossils proves apjjroximate identity of age. 



Equivalency is sometimes shown in an identity of species ; more often in 

 a parallel series of nearly related species ; often by an identity or close rela- 

 tion in the genera or families ; often also in some prominent peculiarity of 

 the various species under a family or class. 



Through a comparison of fossils, it was discovered that the Chalk forma- 

 tion exists on the Atlantic border of the United States, although the region 

 ■contains no chalk ; that the Coal formation of ISTorth America and that of 

 Newcastle, England, belong in all probability to the same geological age ; 

 and so on. 



The progress in life has not consisted in change of species alone. The 

 species of a genus often present, in successive periods, some new feature ; or 

 the higher groups under an order or class some modification, or some new 

 range of genera, so that, even when the species differ, the habit or general 

 characters of the species, or the range of genera or families represented, may 

 serve to determine the era to which a rock belongs, or at least to check off 

 the eras to which it does not belong. "Thus Spirifer, a genus of mollusks, 

 which has a narrow form in the Silurian, has often a very broad form in the 

 course of the Devonian and the Carboniferous ages. Ganoid fishes, which 

 iave vertebrated tails through long ages, have their tails not vertebrated in 

 after time. Trilobites become wholly extinct at a certain epoch in their his- 

 tory. These are examples of a principle availed of in multitudes of cases 

 presenting minor differences. 



Much aid is derived also from the canon brought forward by Agassiz in 

 the first volume of his Poissons Fossiles (1833, pages 208-270), and con- 

 sidered at length in one of the chapters in his Na,tural History of the United 

 States (i. 112, 1857) : that, under the various tribes, the geological succession 

 of species often corresponds in some of the more general characteristics 

 with the succession of phases in the development of living representatives of 

 those tribes. In other words, geological succession and modern embryologi- 

 ■cal succession have near parallelisms. 



Agassiz says, in his work on Fossil Fishes (vol. i, page 169) : " J'ai deja eu plus d'une 

 fois occasion de faire remarquer la grande analogic qu'il y a entre certaines formes 

 embryoniques, qui sont passageres dans le developpement des individus, et les caracteres 

 constans d'une foule de genres de diffe'rentes families, qui n'ont que peu de repre'sentans 

 ■dans la cre'ation actuelle, ou qui sont completement eteints." In his work on the Natural 

 History of the United States, on page 112 of the chapter on "the Parallelism between the 

 geological succession of animals and the embryonic growth of their living representa- 

 tives," Agassiz states the principle as follows : "The phases of development of all living 

 animals correspond to the order of succession of their extinct representatives in past 

 geological time." 



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