104 



ESSAY ON CLASSIFICATION. 



Part I. 



SECTION XXIII. 



LIMITATION OF SPECIES TO PARTICULAR GEOLOGICAL PERIODS. 



Without entering into a discussion respecting the precise limits within ■svhich this 

 fact is true, there can no longer be any doubt, that not only species, but all other 

 groups of animals and plants, have a definite range of duration, as well as individ- 

 uals.^ The limits of tins duration, as fiir as species are concerned, generally coin- 

 cide with great changes in the physical conditions of the earth's surface;^ though, 

 strange to say, most of those investigators who would ascribe the origin of organ- 

 ized beings to the influence of such causes, maintain also, that species may extend 

 from one period to another, wliich implies that these are not affected by such 

 changes.^ 



When considering, in general, the limitation of species to particular geological 

 periods, we might very properly disregard the question of the simiUtaneity of the 

 successive appearance and disappearance of Faunae, as in no way affecting the 

 result of the investigation, as long as it ls 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, stiU considered as 

 identical in several successive periods, is gi-owing smaller and smaller, in proportion 

 as they are more closely compared. I have already shown, long ago, how widely 

 many of the tertiary species, long considered as identical with living ones, differ 

 from them,* and also how different the species of the same family may be, in 

 successive subdivisions of the same great geological formation.^ Hall has come to 

 the same result in his investigations of the fossils of the State of New York.^ 

 Every monograph reduces their number, in every formation. Thus Barrande, who 

 has devoted so many years to the most minute investigation of the Trilobites of 



'^ Compare Sect. XIX. 



2 Elie de Beaujio^x, Eeeherclies siir quelques- 

 unes des Revolutions de la surface du Globe, Paris, 

 1830, 1 vol. 8vo. 



* For indications respecting the occurrence of all 

 species of fossil organized beings now known, consult, 

 Bronn, (H. G.,) Index palrcontologicus, Stuttgardt, 

 1848-49, 3 vols. 8vo. — Okbignt, (A. d',) Prodrome 

 de Paleontologie stratigrapliique universelle etc., 



Paris, 1850, 2 vols. 12mo. — Morris, (J.,) Catalogue 

 of the British Fossils, London, 1854, 1 vol. 8vo. 



* Agassiz, (L.,) Coquilles tertiaires reput^es 

 identiques avec les especes vivantes, Neuchatel, 1845, 

 4to. fig. 



' Agassiz, (L.,) Etudes critiques sur les Mollus- 

 ques fossiles, Neuchatel, 1840-45, 4to. fig. 



* Hall, (J.,) Palaeontology of the State of New 

 York, q. a., p. 23, note 1. 



