THE LAW OF JOHANNSEN. 213 



that arround us a complete vacuum is not met with, does not 

 invalidate Newton's law. And the view that we, who accept 

 the qualitative stability of genes must therefore necessarily 

 hold the fallacious theory that species in nature are pure lines, 

 or are built up of them, is as absurd as the view that a physi- 

 cist who accepts Newton's law must necessarily believe in a 

 universal vacuum on earth. 



Selection in a pure line could theoretically result in a change. 

 This does not mean that selection can change a gene, but that 

 within a pure line genotypic changes are possible, which might 

 furnish the material for such an effect of selection. 



Selection in a pure clone can also result in a change. The fact 

 that so far no effect of selection within pure lines has been 

 noted, whereas it has been observed in clones, is only natural 

 if we remember that mutation in homozygous material means 

 loss of a gene, wheras spontaneous geno- variation in a pure clone 

 can be vegetative segregation, a production of cells lacking in a 

 gene, by cells heterozygous for it. 



It is conceivable that pure clones may at the same time be 

 pure lines, that is, clones may be started from one homozygous 

 individual, just as pure lines. Such clones would presumably 

 be as proof against change by selection as pure lines. But the 

 very fact that vegetative segregation is not comparable with 

 loss-mutation, must warn us against getting mixed up in our 

 terminology. We certainly may not use the term pure line 

 indiscriminately for pure clones. The fact, that we can see 

 vegetative segregation in a group of plants cut from a single 

 heterozygote as well as in the original heterozygote disturb 

 the more usual uniformity, gives us sufficient reason to look 

 for this process as an additional source of hereditable variation, 

 added to mutation. 



It becomes more and more clear, that the ordinary heritable 

 differences between organisms are caused by underlying differ- 

 ences in the set of genes present in their cells, differences to be 

 expressed in presence and absence of individual genes. The 

 permanence of the differences which we observe between ge- 





