564 HISTORY OF GEOLOGY. 



modified, so as to give rise to varieties and races different from what 

 before existed. How different, for instance, is one kind and breed ol 

 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 ? whether the ourang-outang may, by the power of ex- 

 ternal 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 interchangeable, we must suppose the fluctuations of which 

 each species is capable, and which are apparently indefinite, to be 

 bounded by rigorous limits ; whereas, if we allow such a transmuta- 

 tion of species, we abandon that belief in the adaptation of the struc- 

 ture 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 spectacle 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. 

 And the dilemma then presents itself to us anew : either we must 

 accept the doctrine of the transmutation of species, and must suppose 

 that the organized species of one geological epoch were transmuted 

 into those of another by some long-continued agency of natural causes ; 

 or else, we must believe in many successive acts of creation and extinc- 

 tion of species, out of the common course of nature ; acts which, there- 

 fore, we may properly call miraculous. 



This latter dilemma, however, is a question concerning the facts 

 which have happened in the history of the world ; the deliberation 

 respecting it belongs to physical geology itself, and not to that subsi* 

 diary science which we are now describing, and which is concerned 

 only with such causes as we know to be in constant and orderly 

 action. 



The former question, of the limited or unlimited extent of the 

 modifications of animals and plants, has received full and careful consi- 

 deration from eminent physiologists ; and in their opinions we find, I 

 think, an indisputable preponderance to that decision which rejects 

 the transmutation of species, and which accepts the former side of 

 the dilemma ; namely, that the changes of which each species is suscep- 



