REDUCTION OF VARIABILITY. 109 



the selection which is responsible for the purity, but only the 

 colonisation from one plant chosen by the selection. Any other 

 cause, which isolates the progeny of one plant from the mixture 

 will have the same effect, will cause the same purity. Hallett 

 and de Vilmorin introduced into plant-breeding the principle of 

 construing in every generation from one single individual, 

 where the plants were self- fertilized. In these plants, as well as 

 in asexually propagated ones, the effect of choosing one single 

 individual is a complete elimination of all genetic variability. 

 Once the plant with the most suitable genotype is found, the 

 further work consists merely of propagating it, and keeping its 

 progeny free from admixture. The production of new wheats or 

 new sugar-canes, by this process of choosing one single individ- 

 ual, illustrates better than any other process, the fact, that 

 it is the isolation which here produces purity and not the select- 

 ion. It has been demonstrated repeatedly, that any individual 

 in such plants will have a uniform descendance, and on the 

 other hand it is easy to show that mass-selection in the same 

 material will be comparatively ineffective to produce a high 

 quality, and a great uniformity. In comparing mass-selection 

 and individual selection in the work of the plant-breeders we 

 commonly compare two processes, which are more unlike than 

 is implied in the terms. The system of selecting one individual 

 plant, either as such, or after examination of its progeny (Vil- 

 morin's method) is generally followed with self -fertilized plants, 

 with rice, wheat, oats, tomato, pea, whereas mass-selection is 

 the most common method of breeding allogamous plants. 

 This must be mainly due to conservatism. Wheat is still bred 

 essentially in the identical way in which de Vilmorin started 

 breeding it, and sugar-beets are still bred in the majority of 

 instances in the way in which he originated them. Only com- 

 paratively recently the plant-breeders have made the discovery, 

 that even in habitually cross-fertilized plants the variability 

 in the off-spring of one plant is very much smaller than that of 

 the mixed off-spring of a greater number of individuals. And so 

 individual selection has under various names been introduced 



