202 THE STORY OF BIRD-LIFE. 



and by which they are distinguished from their 

 neighbours. 



This process of creating new forms by modify- 

 ing old ones we call the process of "Artificial 

 Selection." But, mark that whatever changes 

 have taken place have been changes wrought in 

 already existing structures. There have been 

 no sudden introductions of structures hitherto 

 non-existent or foreign to their nature as — say, 

 pigeons. The rock-dove from which each par- 

 ticular breed of pigeon is derived was already 

 & delicately organised, and highly specialised 

 animal, so that any changes which could possibly 

 be effected in its form must necessarily proceed 

 5ilong certain lines by the exaggeration of al- 

 ready existing structures. It would have been 

 impossible for instance to change it into a form 

 resembling a hawk or a crow. 



It sometim.es happens with the pedigrees of 

 distinguished people that the further back we 

 trace their ancestry, the more humble and 

 obscure does it become. So is it with animals 

 and plants. The exact manner in which these 

 have come by their present forms, and what 

 their ancestors were like, is the puzzle which 

 men of science are trying to unravel. The 

 machinery which has been largely responsible 

 is at work all round us, and has been going on 

 since the beginning of living things. 



"For though the mills of God grind slowly, 

 Yet they grind exceeding small." 



We are daily sifting these fragments in the 

 hope of finding now and again a few that will fit 



