164 EVOLUTION AND DOGMA. 



between them, if they come into competition with 

 each other, than between the species of distinct 

 genera. We see this in the recent extension over 

 the United States, of one species of swallow, having 

 caused the decrease of another species. The recent 

 increase of the missel-thrush in parts of Scotland has 

 caused the decrease of the song-thrush. How fre- 

 quently we hear of one species of rat taking the place 

 of another species under the most different climates ! 

 In Russia, the small, Asiatic cockroach has every- 

 where driven before it its great congener. In Aus- 

 tralia, the imported hive-bee is rapidly exterminating 

 the small, stingless, native bee. One species of char- 

 lock has been known to supplant another species ; 

 and so in other cases. We can dimly see why com- 

 petition should be most severe between allied forms 

 which fill nearly the same place in the economy of 

 nature ; but probably in no one case could we pre- 

 cisely say why one species had been victorious over 

 another in the great battle of life." ' 



Variations and the Formation of Fossiliferous Deposits. 



Then again, it must be observed that it is not 

 probable that variation has been going on at a uniform 

 rate during the long course of the life-history of the 

 earth. On the contrary, it is more likely that long 

 periods of stability have alternated with brief periods 

 of disturbance of greater or less extent. During the 

 former periods specific forms would experience com- 

 paratively little change, whereas, during the latter, 

 variations would rapidly accumulate and be strongly 



1 "The Origin of Species," vol. I, pp. 93 and 94. 



