IMMUTABILITY OF SPECIES. 81 



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 species 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 doubtful, 

 whether their representatives in two successive epochs 

 are descendants one of the other, as we have already 

 strong evidence in favour of the separate origin of the 

 representatives of the same species in separate geo- 

 graphical areas. 1 The case of closely allied but different 

 species occurring in successive periods, yet limited re- 

 spectively to their epochs, affords, in the course of time, a 

 parallel to the case of closely allied, so-called, representa- 

 tive species occupying different areas in space, which no 

 sound naturalist would now suppose to be derived one 

 from the other. There is no more reason to suppose 

 species equally allied, following one another in time, to 

 be derived one from the other ; and all that has been 

 said in preceding paragraphs respecting the differences 

 observed between species occurring in different geo- 

 graphical areas applies with the same force to species 

 succeeding each other in the course of time. 



When domesticated animals and cultivated plants are 

 mentioned as furnishing evidence of the mutability of 

 species, the circumstance is constantly overlooked, or 

 passed over in silence, that the first point to be es- 

 tablished respecting them, in order to justify any in- 

 ference from them against the fixity of species, would be 



dermes vivans et fossiles; Neuchatel, 4to., atlas, fol. 



1838-42, 4 nos., 4to., fig. AGASSIZ l See Sect. 10, where the case of 



(L.), Recherches sur les Poissons fos- representative species is consider- 



siles; Neuchatel, 1833-44, 5 vols., ed. 



G 



