62 ESSAY ON CLASSIFICATION 



and this regular succession should extend only as far north as Lake 

 Ogeechobee, for two degxees of latitude, this would give about two 

 hundred thousand years for the period of time which was necessary 

 for that part of the peninsula of Florida which lies south of Lake 

 Ogeechobee to rise to its present southern extent above the level of 

 the sea, and during which no changes have taken place in the char- 

 acter of the animals of the Gulf of Mexico. ^^ 



It is very prejudicial to the best interests of science to confound 

 questions that are entirely different, merely for the sake of support- 

 ing a theory; yet this is constantly done, whenever the question of 

 the fixity of species is alluded to. A few more words upon this point 

 will, therefore, not be out of place here. 



I will not enter into a discussion upon the question whether any 

 species is found identically the same in two successive formations, as 

 I have already examined it at full length elsewhere,^^ and it may be 

 settled finally one way or the other, without affecting the proposition 

 now under consideration; for it is plain, that if such identity could 

 be proved, it would only show more satisfactorily how tenacious spe- 

 cies are in their character to continue to live through all the physical 

 changes which have taken place between two successive geological 

 periods. Again, such identity once proved would leave it still doubt- 

 ful whether their representatives in two successive epochs are de- 

 scendants one of the other, as we have already strong evidence in 

 favor of the separate origin of the representatives of the same species 

 in separate geographical areas. ^^ The case of closely allied but differ- 

 ent species occurring in successive periods, yet limited respectively 

 in their epochs, affords in the course of time a parallel to the case of 

 closely allied, so-called, representative species occupying different 

 areas in space, which no sound naturalist would suppose now to be 

 derived one from the other. There is no more reason to suppose 

 equally allied species following one another in time to be derived 

 one from the other; and all that has been said in preceding para- 

 graphs respecting the differences observed between species occurring 



*" According to facts recently observed and alluded to above, double that time at 

 least has elapsed since their first appearance in these waters. 



^'Agassiz, itudes critiques sur les Molliisques fossiles (4to., Neuchatel, 1831-1845); 

 Monographies d'Echinodermes vivans et fossiles (4 nos., 4to., Neuchatel, 1838-1842); 

 Recherches sur les Poissons fossiles (5 vols., Neuchatel, 1833-1843). 



**See Sect, x, where the case of representative species is considered. 



