DURATION OF SPECIES. 355 



been kept distinct for many generations." Whether 

 this reinforcement is a complete preventive of de- 

 crepitude in species, or only a palliative, is more than 

 we can determine. If the latter, then existing species 

 and their derivatives must perish in time, and the 

 earth may be growing poorer in species, as M. Naudin 

 supposes, through mere senility. If the former, then 

 the earth, if not even growing richer, may be expected 

 to hold its own, and extant species or their derivatives 

 should last as long as the physical world lasts and 

 affords favorable conditions. General analogies seem 

 to favor the former view. Such facts as we possess, 

 and the Darwinian hypothesis, favor the latter. 



