156 ESSAY ON CLASSIFICATION. 



to particular geological periods, we might very properly 

 disregard the question of the simultaneousness of the suc- 

 cessive appearance and disappearance of Faunae, as in no 

 way affecting the result of the investigation, as long as it 

 is universally conceded that there is no species, known 

 among the fossils, which extends through an indefinite 

 series of geological formations. Moreover, the number of 

 the species, still considered as identical in several suc- 

 cessive periods, is growing smaller and smaller, in pro- 

 portion as they are more closely compared. I have already 

 shown, long ago, how widely many of the tertiary species, 

 .generally considered as identical with living ones, differ 

 from them, 1 and also how different the species of the same 

 family may be in successive subdivisions of the same great 

 geological formation. 2 Hall has come to the same result, 

 in his investigations of the fossils of the State of New 

 York. 3 Every monograph reduces their number in each 

 formation. Thus Barrande, who has devoted so many 

 years to the most minute investigation of the Trilobites 

 of Bohemia, 4 has come to the conclusion that their species 

 do not extend from one formation to the other ; D'Or- 

 bigny 5 and Pictet 6 have come to the same conclusion for 

 the fossil remains of all classes. It may well be said, that, 

 as fossil remains are studied more carefully in a zoological 

 point of view, the supposed identity of species, in different 

 geological formations, gradually vanishes more and more ; 

 so that the limitation of species in time, already ascer- 

 tained in a general way, by the earlier investigations of 



1 AGASSIZ, (L.,) Coquilles tertiaires 4 BARKAM)E,Systemesilurien, etc., 

 reputees identiques avec les especes q. a. ; see, also, my Monographies 

 vivantes ; Neuchatel, 1845, 4to. fig. d'Echinodermes, q. a., p. 80. 



2 AGASSIZ, (L.,) Etudes critiques 5 D'OKBIGNY, Paleoutologie Fran- 

 sur les Mollusquesfossiles; Neuchatel, gaise, q. a., p. 143. 



1845-46, 4to. fig. G PICTET, Traite de Paleontologie, 



3 HALL, (J.,) Palaeontology of the etc., q. a., p. 144, note 2. 

 State of New York, q. a, p. 32, note 1. 



