148 MENDELISM chap. 



individuals, and the recognition of this will save the 

 breeder much labour, and enable him to fix his 

 varieties in the shortest possible time. 



Such cases as these of the sweet-pea throw a 

 fresh light upon another of the breeder's conceptions, 

 that of purity of type. Hitherto the criterion of a 

 ■' pure-bred " thing, whether plant or animal, has 

 been its pedigree, and the individual was regarded 

 as more or less pure bred for a given quality accord- 

 ing as it could show a longer or shorter list of 

 ancestors possessing this quality. To-day we realise 

 that this is not essential. The pure-bred picotee 

 appears in our F^ family though its parent was a 

 purple bicolor, and its remoter ancestors whites for 

 generations. So also from the cross between pure 

 strains of black and albino rabbits we may obtain in 

 the Fg generation animals of the wild agouti colour 

 which breed as true to type as the pure wild rabbit 

 of irreproachable pedigree. The true test of the 

 pure breeding thing lies not in its ancestry but in 

 the nature of the gametes which have gone to its 

 making. Whenever two similarly constituted gametes 

 unite, whatever the nature of the parents from which 

 they arose, the resulting individual is homozygous 

 in all respects and must consequently breed true. 

 In deciding questions of purity it is to the gamete, 

 and not to ancestry, that our appeal must hence- 

 forth be made. 



Improvement is after all the keynote to the 

 breeder's operations. He is aiming at the produc- 

 tion of a strain which shall combine the greatest 

 number of desirable properties with the least number 

 of undesirable ones. This good quality he must 



