i82 PLANT-BREEDING 



number of cases, it must be procured artificially. The pro- 

 duction and augmentation of variability is therefore to be 

 considered as a second point of almost equal importance to 

 the selection itself. It may be either plain and simple or 

 intricate. In the simple cases it is often possible to trace the 

 whole line of the work which has led to the ultimate result. 

 In the more intricate cases, however, the breeders ' purpose 

 is simply to increase the material for their selection as largely 

 as possible. All means conducive to this are combined and 

 the scientist finds himself at a loss in trying to discern the real 

 causes from amidst the chaos. 



For tliis reason, I shall Hmit my present discussion to the 

 more clear and simple instances, leaving the complex cases 

 for my next chapter. Of course, a sharp hmit cannot be ex- 

 pected, and an arbitrary choice is unavoidable. 



The investigations of the past decades have led to the rec- 

 ognition of various kinds of variability, and all further re- 

 search has to start from these distinctions. The breeders, on 

 the other hand, are not concerned with these divisions and 

 simply consider the variability of their plants as such. But 

 variability embraces on one side the existence of a larger or 

 lesser range of different types, and on the other hand the 

 actual transformation of one form into another. It is evident 

 that a scientist wishes to know in each case to which of these 

 two types an observed fact of variability belongs, while this 

 distinction may be indifferent to the practical man. In some 

 cases, however, it is not at all indifferent. I mean those im- 

 provements which have to produce races that can be multi- 

 pHed by seed. All the varieties of cereals, most of the other 

 agricultural crops, and many kinds of garden plants belong 

 to this group. Here it is manifest that only inheritable vari- 

 ations are of consequence, or strictly speaking, only those that 

 will be repeated by generations raised from seeds. 



If, however, a new type is not intended to be reproduced 



