210 THE LAW OF JOHANNSEN. 



self-fertilization out of one homozygous individual. A pure clone 

 is a group of individuals which originates by an asexual 

 process from one individual. Now, ordinarily, changes in the 

 genotype of individuals of a pure line are rare indeed, and so 

 are corresponding changes in the set of genes carried by the 

 members of a pure clone. And therefore, ordinarily selection 

 within a pure line or a pure clone, must be ineffective. But there 

 are certainly exceptions to the rule, that selection in pure lines 

 and in pure clones is ineffective, simply because there are in- 

 stances in which within pure lines quite apart from any select- 

 ion, quasi-spontaneous changes in the set of genes transmitted 

 have been noticed. And a little more frequently, analogous 

 changes have been noted in pure clones. 



Johannsen has observed such a mutation, the spontaneous 

 loss of a gene from a bud in beans. Nilsson Ehle has observed a 

 few instances of real loss-mutation in oats and wheat. In clones, 

 we occasionally meet instances of a kind of vegetative segre- 

 gation. We find, that an individual heterozygous for a certain 

 gene may produce buds in which this gene is lacking. Very 

 many horticultural sorts of Dahlia, Chrysanthemum and Azalea 

 have originated in this way. That we are here concerned with a 

 vegetative segregation and not ordinarily with loss-mutation, 

 comes to light in those instances in which a plant which in an 

 asexual way produced individuals unlike itself, is tested by 

 self-fertilization. The results of Salaman with potatoes are very 

 striking. Some plants with elongated tubers produced occasion- 

 al round tubers, which in their turn would produce only 

 round ones. Self -fertilized, the plants with the elongated tubers, 

 gave a minority of off-spring with round tubers, thus showing 

 their heterozygous nature. 



A good way to bring out the relation between the law of 

 Johannsen and the effect of selection, is to compare these ef- 

 fects of selection, to the behaviour of falling bodies, and the 

 relation between this behaviour and Newton's law of gravity. 

 The attraction between bodies is in direct proportion to their 

 mass and in inverse proportion to the square of the distanc^ 



