A THEORY OF RETROGRESSION $7 



Any breeder could tell us. The prize variety would 

 swiftly revert to the ordinary type. Each succeed- 

 ing generation would be inferior to the preceding 

 generation, till the ancestral type was at last re- 

 produced. The more rapid the previous evolution, 

 the more rapid would be the subsequent reversion. 

 When several prize varieties, e.g: pigeons, have 

 been evolved from a common ancestral stock, we 

 get, if we interbreed them, the acme of non- 

 selection — artificial non-selection. In such a case 

 the common stock reappears. Thus the wild blue 

 pigeon, the Columba Livia, reappears as a result 

 of interbreeding the many varieties of tame pigeons. 

 Our prize breeds of animals have been evolved 

 under very stringent selection. But many of our 

 garden plants have been evolved under selection a 

 thousandfold more severe. Most plants are capable 

 of bearing annually innumerable offspring, among 

 which we are able to exercise very stringent selec- 

 tion. Moreover, all the plants we have most rapidly 

 evolved are capable of being propagated by means 

 of slips and grafts. A plant reared from a slip, 

 sucker, or graft is not a new individual ; it is merely 

 a detached part of an old individual. Roughly 

 speaking, the process of the evolution of these 

 plants is as follows : The gardener sows a number 

 of seeds from a good plant. He chooses the best 

 of the resulting offspring, destroying the others. 



