246 A STUDY IN HEREDITY 



appreciable change of type. Thus, under ordinary circumstances, 

 if a man reverted to any particular ancestor of a thousand years 

 ago, no one would recognise to what the change of type was due. 

 Not only would the change be too slight, but the observer would 

 need to have a knowledge of the ancestral form, and such know- 

 ledge is usually impossible. Sometimes, however, recognisable 

 reversion does occur even among such beings. Thus a man may 

 resemble the portrait of some far-away ancestor ; or, again, the 

 progeny of an ordinary pair of horses may exhibit the zebra-like 

 stripes of a remote ancestor. It is not, however, among complex 

 beings, slowly evolved in every particular, that we must seek our 

 proofs. We must turn to plants and animals that have under- 

 gone swift evolution in some one particular, and this, so far as I 

 know, occurs only under stringent Artificial Selection. For 

 Natural Selection, having care for many characters, results in 

 but slow evolution, — but Artificial Selection, having care for one 

 or only a few characters, results in much swifter evolution. 

 Supposing, then, we take any breed of domesticated animals or 

 cultivated plants, and, after choosing the finest specimens, hence- 

 forward breed indiscriminately from these and their descendants, 

 what then happens ? It is notorious that under such circum- 

 stances cessation of selection is marked by a reversion towards 

 the ancestral type — a reversion swift in proportion to the swiftness 

 of the antecedent evolution. Thus, without continued stringent 

 selection the speed of race-horses cannot be maintained ; they 

 tend to lose their special characters, and revert to the ordinary 

 horse. The same is true of all other prize breeds. Again, careful 

 breeding from ordinary horses readily evolves a speedier race ; 

 for the offspring of ordinary horses, in many instances, surpass 

 the parents. But, in proportion to the success of the breeder, 

 further improvement grows continually more and more difficult, 

 till, at length, evolution practically reaches a standstill. Improve- 

 ment thereafter is very slow indeed. For this reason it is now 

 very difficult to improve our breed of race-horses. The offspring 

 of a pair of the finest animals are, in the great majority of cases, 

 inferior to their parents, and, therefore, practically all that the 



