BI-PARENTAL REPRODUCTION 77 



tion. In other words, reversion towards the ancestral type is 

 swift in inverse proportion to the number of generations that 

 intervene between the prize variety and the type wheoce it was 

 derived. Thus, without stringent selection, the speed of race- 

 horses, which depends on structural peculiarities, cannot be main- 

 tained. They tend to lose their special characters and to revert 

 towards the ordinary horse. The same is true of all the prize 

 breeds. 1 Thus large prize dogs tend to produce smaller off- 

 spring, while small breeds are with difficulty kept small. 

 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. 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 instances, 

 inferior to their parents, and consequently the most, or almost 

 the most, that stringent selection is now able to achieve is the 

 preservation, not the improvement, of the race. 2 It is plain, 

 therefore, that, owing to the increasing tendency towards 

 reversion, rapid evolution quickly slows down, till, even in the 

 presence of stringent selection, it practically ceases. 



134. But perhaps the most striking evidence is 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. In effect the most 

 favourable individual of a species is chosen and multiplied 

 by means of slips, the rest of the species being eliminated ; 



1 "If a considerable number of improved cattle, sheep, or other 

 animals of the same race were allowed to breed freely together, with no 

 selection, but with no change in their conditions of life, there can be 

 no doubt that after a score or a hundred generations they would be very 

 far from excellent of their kind." (Animals and Plants, vol. ii., p. 225.) 

 " When selection is suspended rapid deterioration (from the fancier's 

 standpoint) is the inevitable result. If, e. g., a number of pigeons, good 

 specimens of a distinct breed, are isolated and left unmolested for a few 

 years, they rapidly degenerate, i. e. they lose their show points (be they 

 freaks, frills, ruffs or metallic tints) and re-assume the more fixed ances- 

 tral characters." (Professor J. Cossar Ewart. Presidential address to the 

 Zoological Section of the British Association, 1901.) 



2 "Two years ago thirty-two yearlings were sold for 51,250 guineas. 

 These thirty-two yearlings are represented by two winners of five races, 

 Florio Rubbatino, and La Reine, who have contributed about 2000 to 

 the total cost ; and there is not, as far as can be known, a single one of 

 the remaining thirty with any prospect of making a race-horse." 

 (Quoted from The Times, December 27th, 1897, by Sir Walter Gilbey. 

 Race-horses, p. 6.) 



