A THEORY OF RETROGRESSION 55 



which parts, previously useful to wild creatures, 

 become useless. For instance, ages must have 

 elapsed while the dodo changed from an aerial to 

 a ground bird. 



But man is able to observe very closely the 

 changes among the plants and animals he has 

 under his destructive care. He eases the 

 stringency of selection in many directions. Char- 

 acters highly useful in the wild state become 

 useless, or less useful. He is thus able to increase 

 enormously the stringency of selection in some 

 particular direction, and thereby cause rapid evolu- 

 tion in this or that chosen character. From his 

 race-horses, for example, he asks only speed and 

 some endurance, and sacrifices almost all else to 

 the evolution of these characters. He cares little 

 if his finest race-horses grow somewhat defective 

 in sight and smell and hearing. From his draught- 

 horses he asks only weight and strength ; from his 

 cattle only an abundance of food ; from each of 

 his pets only one or two fanciful qualities ; from 

 his plants only excellence in fruit, or flower, or 

 leaf. Every breeder is thus, during the short span 

 of his life, able to observe great changes in plants 

 and animals — changes produced by the concentra- 

 tion of effort on a single object. 



It is a known fact that characters that have long 

 been present in a species degenerate very slowly 



