50 



as starting points for new races cannot depend exclusively upon apparent 

 morphological differences. 



Influence of I n commenting upon the influence of mass-selection, in a case such as 



mass-selec- this ^ Nilsson-Ehle (42 p. 128) says: "Were a mass-selection of plants over 

 125 c.m. in height to be made from this old mixed sort, plants from types 

 which normally produce a short straw would be taken as would also those from 

 types which normally produce a taller growth. The latter would naturally 

 preponderate whereby a certain advance in the desired direction would likely 

 be made." It is quite possible, however, that an advance in one direction 

 may be made at the expense of some more valuable quality, hence the 

 danger which is associated with this form of selection. 



The Origin of Aberrant Forms as Quantitative Hereditary Variations. 



Apart from the mass of apparently related individuals which go to make 

 up the greater part of a plant population, there may occasionally arise 

 strange forms which at first sight do not seem traceable to any definite 

 parentage. There may arise bearded heads of wheat in a bald sort, brown- 

 chaffed individuals in a white-chaffed sort, white-kernelled forms in a red- 

 kernelled sort, etc. In oats, white and grey kernelled individuals have been 

 found in black-grained sorts and vice versa, while side oat types have been 

 found in sorts characterized by spreading panicles. Formerly these aberrant 

 forms were commonly regarded as Atavists or Reversions, being looked upon 

 as the sudden reappearance of certain ancestral characters. More recently 

 they have received the name Mutation. Experience at Svalof and elsewhere 

 has shown that the majority of these so-called novelties which thus suddenly 

 appear in cultivated crops may be produced artificially by cross-hybridiza- 

 tion and may therefore be regarded in most cases, simply as new combina- 

 tions of already existing units. Apart from the great scientific interest which 

 surrounds the appearance of these aberrant individuals there is an interest 

 for the practical breeder which cannot be denied. If these forms represent 

 mutations by which apparently new characters are suddenly acquired, it 

 would clearly be the breeder's main duty to watch carefully for their appear- 

 ance in his fields with a view to isolating and propagating them and perchance 

 obtaining something better than the old sort. On the other hand, if they 

 represent the results of natural crossings between different sorts, as they are 

 now believed to do, it is of much less importance to spend time in seeking for 

 things which can be produced artificially with much greater assurance of 

 obtaining an advance. Thus where formerly, striking natural crosses found 

 in the experimental plots at Svalof were eagerly isolated and studied they 

 are now very largely ignored unless the marks by which they are characterized 

 point to a certain parentage of known value. Instead it is preferred to make 

 crossings artificially between known sorts whose values have already been 

 proven. 



