102 ESSAY ON CLASSIFICATION 



logical formations. Moreover, the number of the species, still con- 

 sidered as identical in several successive periods, is growing 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 for- 

 mation. Thus Barrande, who has devoted so many years to the most 

 minute investigation of the Trilobites of Bohemia, has come to the 

 conclusion that their species do not extend from one formation to 

 the other; d'Orbigny and Pictet 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 van- 

 ishes gradually more and more; so that the limitation of species in 

 time, already ascertained in a general way by the earlier investiga- 

 tions of their remains in successive geological formations, is circum- 

 scribed step by step within narrower, more definite, and also more 

 equable periods. Species are truly limited in time, as they are limited 

 in space, upon the surface of the globe. The facts do not exhibit a 

 gradual disappearance of a limited number of species and an equally 

 gradual introduction of an equally limited number of new ones; but 

 on the contrary, the simultaneous creation and the simultaneous de- 

 struction of entire faunae, and a coincidence between these changes 

 in the organic world and the great physical changes our earth has 

 undergone. Yet it would be premature to attempt to determine the 

 extent of the geographical range of these changes, and still more 

 questionable to assert their synchronism upon the whole surface of 

 the globe in the ocean and upon dry land. 



To form adequate ideas of the great physical changes the surface 

 of our globe has undergone, and the frequency of these modifications 

 of the character of the earth's surface, and of their coincidence Avith 

 the changes observed among the organized beings, it is necessary to 



^" Agassiz, Coquilles tertiares reputees identiqucs ax'cc les espcces I'wantes (Neuchatel, 

 1845); and £tudes critiques sur les Mollusques fossiles (4 vols., NeuchStel, 1840-1845). 

 ^^ Palceontology of New York. [See above, n. 33.] 



