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 typein 
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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