1898 j A THEORY OF RETROGRESSION 399 



evolutionary variation in other particulars. It is slight because, 

 since such species have evolved but slowly, reversion to a not very 

 remote ancestor does not result in any appreciable change of type. 

 Thus, under ordinary circumstances, if a man reverted in any parti- 

 cular to an 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 knowledge 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 undergone swift evolution in some one particular, and 

 this, so far as I know, occurs only under stringent Artificial Selec- 

 tion. For Natural Selection, having care for many characters, re- 

 sults in but slow evolution ; but Artificial Selection, having care for 

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

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

 vated plants, and, after choosing the finest specimens, henceforward 

 breed indiscriminately from these and their descendants ; what then 

 happens ? It is notorious that under such circumstances cessation 

 of selection is marked by a reversion towards the ancestral type, a re- 

 version 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. 

 Improvement thereafter is very slow indeed. For this reason it is 

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

 spring of a pair of the finest animals are in the great majority of 

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

 most stringent selection is now able to achieve is to preserve, not to 

 improve, the race. It is, therefore, plain that, owing to the increas- 

 ing tendency towards reversion, rapid evolution quickly slows down, 

 till, even in the presence of stringent selection, it practically ceases. 

 But perhaps the most striking proofs of the present theory are 

 furnished by certain cultivated plants (for instance the apple), which 

 are usually propagated by means of slips or suckers — that is, by 

 detached portions of the individual. Practically speaking, the most 



