278 THE GENESIS OF SPECIES. [Chap, XI. 



mena it fails to explain, and on account of certain otlier 

 phenomena wliich seem actually to conflict with tliat 

 theory. AVe have seen that though the laws of nature are 

 constant, yet some of the conditions which determine 

 specific change may be exceptionally aljsent at the present 

 epoch of the world's history ; also that it is not only 

 possible, but highly probable, that an internal power or 

 tendency is an important if not the main agent in pro- 

 ducing the manifestation of new species on the scene of 

 realized existence, and that in any case, from tlie facts of 

 homology, innate internal powers to the full as mysterious 

 must be accepted, whether they act in specific origination 

 or not. Besides all this, we have seen that it is probable 

 that the action of this innate power is stimulated, evoked, 

 and determined by external conditions, and also that the 

 same external conditions, in the shape of " Natural Selec- 

 tion," play an important })art in the evolutionary process : 

 and finallv, it has been affirmed that the view here advo- 

 cated, while it is supported by the facts on which Dar- 

 winism rests, is not open to the objections and difficulties 

 which oppose themselves to the reception of " Natural 

 Selection" as the exclusive or even as the main agent in 

 the successive and orderly evolution of organic forms in 

 the (jcncsis of species. 



