156 EVOLUTION 



will be changed; some will probably become 

 extinct; and these changes will seriously 

 affect the others. Immigration of new forms 

 might also occur, with further serious dis- 

 turbance; or, where this is impossible, there 

 will be places in the economy of Nature 

 which might be better filled up. In such 

 cases slight changes in structure or habit 

 which in any way favoured the individuals 

 of any species, by adapting them better to 

 their altered conditions, would tend to be 

 preserved, and natural selection would have 

 free scope for its work of improvement. 

 Moreover, changed conditions increase varia- 

 bility. 



As man produces great results by his 

 artificial selection, what may not natural 

 selection effect? Man selects only for his 

 own purposes, Nature for the good of the 

 creature itself; man on the more external 

 characters (he has become more adventurous 

 since Darwin's day), Nature on the whole 

 machinery of life; man irregularly and im- 

 perfectly for a short time, Nature by con- 

 sistent accumulation during whole geological 

 periods. Natural selection is daily and 

 hourly scrutinizing, throughout the world, 

 the slightest variations, rejecting those that 

 are bad, preserving and adding up all that are 

 good, silently and insensibly working, when- 



