PURE LINES 257 



true to type. With plants the method of procedure depends upon the 

 details of reproduction in the species under consideration. For example, 

 corn is naturally cross-fertilized but is also self-fertile, while the common 

 sunflower is self-sterile and so must always be cross-fertilized. With 

 such plants as the sunflower, then, the procedure will be as with animals 

 and the length of time required to produce approximately pure lines will 

 depend upon three things: (1) the number of genetic factors for which 

 each of the selected individuals is heterozygous; (2) the number of genetic 

 factors with respect to which the two selected individuals differ; (3) the 

 number of chromosomes in the species. The specific chromsome number 

 is an important consideration because of its direct relation to the number 

 of linked character groups or in other words to the possible number of 

 freely assorting pairs of factors. Sufficient has been said concerning the 

 comparative ease of isolating pure lines from populations of autogamous 

 species and the relative difficulty of obtaining pure lines from allogamous 

 species to make it clear that the material under consideration is of the 

 highest importance in all critical discussions of the effect of selection 

 within pure lines. Finally, it is to be noted that a vegetatively pro- 

 pagated phenotype may or may not be a pure line according to its 

 genetic constitution. A group of individuals thus propagated is known 

 as a clone. In strictly allogamous species a clone would hardly ever be 

 homozygous. 



The Effect of Selection Within Pure Lines. -There is now con- 

 siderable evidence in support of the theory that selection within a pure 

 line is without effect. This evidence comes from the results of practical 

 breeding as well as scientific investigations of certain autogamous species 

 of plants, such as wheat, oats and barley; also from thoroughgoing re- 

 search on a few allogamous species, especially on certain insects and pro- 

 tozoa, particularly paramecia. The constant maintenance of head type in 

 wheat is strikingly portrayed in Fig. 106, which shows two heads from each 

 of four varieties which were first isolated by Louis de Vilmorin between 

 1836 and 1856. The plants according to Vilmorin were found to be 

 identical in all respects "although separated by an interval of 50 years 

 during which annual selection had been continued. This fixity is shown 

 not only in the characters of the ear but also in all the other characters 

 of the plant even that of precocity, which would appear to be most 

 dependent on climate." The use of this case as evidence in support 

 of the pure-line theory has been criticised upon the ground that the selec- 

 tion practised had for its purpose the preservation rather than the altera- 

 tion of the type. But from the experience of many investigators and 

 breeders we may safely conclude that within true pure lines selection is 

 without effect on the type unless mutations occur. After subjecting a 

 variety of barley known as Glorup to plus and minus selection for eight 



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