624 HISTORY OF GEOLOGY. 



upon their constitution, be greatly modified, so as 

 to give rise to varieties and races different from 

 what before existed. How different, for instance, is 

 one kind and breed of dog from another! The 

 question, then, is, whether organized beings can, by 

 the mere working of natural causes, pass from the 

 type of one species to that of another ? whether the 

 wolf may, by domestication, become the dog ? whe- 

 ther the ourang-outang may, by the power of exter- 

 nal circumstances, be brought within the circle of the 

 human species ? And the dilemma in which we are 

 placed is this ; that if species are not thus inter- 

 changeable, we must suppose the fluctuations of 

 which each species is capable, and which are appar- 

 ently indefinite, to be bounded by rigorous limits ; 

 whereas, if we allow such a transmutation of spe- 

 cies, we abandon that belief in the adaptation of 

 the structure of every creature to its destined mode 

 of being, which not only most persons would give 

 up with repugnance, but which, as we have seen, 

 has constantly and irresistibly impressed itself on 

 the minds of the best naturalists, as the true view 

 of the order of the world. 



But the study of geology opens to us the spec- 

 tacle of many groups of species which have, in the 

 course of the earth's history, succeeded each other 

 at vast intervals of time ; one set of animals and 

 plants disappearing, as it would seem, from the face 

 of our planet, and others, which did not before 

 exist, becoming the only occupants of the globe. 



