170 MUTATION. 



all having the same effect upon a certain stage of the develop- 

 ment of the young fruit, was three. But it is easy to conceive of 

 cases where four or six or more genes, which in their action 

 upon the rest of the development may act quite differently, 

 have the same influence upon one certain point. Any day 

 Shull may hit upon Capsella plants lacking A, B as well as C, 

 and still with normal capsules, because of a possession of a 

 gene D, or two genes, D and E. 



Now in all such instances, wild-growing heterozygotes may 

 be taken up into our experimental gardens, which when selt- 

 fertilized will produce a very small proportion of plants with a 

 recessive new character. If the number of parallel genes is 

 large, and the number of plants grown in each generation is 

 small, it may take two or several generations for the novelty 

 to be seen. In a case of five genes, only one plant in every 1024 

 descendants of a heterozygote would show the new recessive 

 character. 



The required test-matings which, as we have just seen, are 

 very difficult to perform, and which it is sometimes impossible 

 to make, as in annual plants, can nevertheless hardly ever be 

 dispensed with if we want to make sure of a case of mutation. 

 Mutations in pure lines however, can be accepted as such. The 

 difficulty here lies in the fact that it is hardly possible to be 

 sure, that a group of organisms constitutes a pure line. If we 

 hold to Johannsen's definition, a pure line is a group of plants 

 grown by repeated self-fertilization out of one individual, pure 

 for all its genes. The rapid reduction of geno-variability caused 

 by self-fertilization makes it probable, that nearly always the 

 descendants of one individual plant of wheat or oats or pea is a 

 real pure line. And, therefore, spontaneous production of nov- 

 elties in such material, as observed by Johannsen and by 

 Nilssen Ehle have great significance, although it is not strictly 

 defendable even in this material to speak of pure lines. And we 

 should, to avoid confusion, always protest against the abuse of 

 the term in those cases where authors denote by "pure line' 

 simply pure-bred material, such as self-fertilized series of plants, 



