260 GENETICS IN RELATION TO AGRICULTURE 
biotypes can be differentiated from one another so that they “breed 
true.” How these distinct biotypes originate will be considered in the 
following chapter, the fact that they exist is the chief consideration here. 
The effect of ‘‘mass”’ selection in causing temporary changes in heteroge- 
neous varieties of plants and races of animals is easily understood by 
the aid of the diagram shown in Fig. 108. The area within the large 
curve represents a mixed population or phenotypically similar group 
containing a number of distinct genotypes indicated by the small curves 
A-Z. Every genotype has its own variation curve and is distinct from 
each of the others, but they intergrade with each other so completely 
that the population appears as an entity. Now if one should select 
individuals from either extreme of the population, say at 90 or 70, it is 
clear that such individuals might belong to any one of four or five geno- 
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10), 
Fic. 108.—Schematic diagram showing the relation of a population to the biotypes 
composing it, or of a phenotype, to the genotypes or pure lines within it. (After Lang 
from Goldschmidt.) 
types. If selection in the same direction were continued a strain would 
be established with a mode distinct from the mode of the original popu- 
lation. These strains could be maintained by continual selection 
and in time a single genotype might be isolated when selection would 
be said to have changed the type permanently. But selection changed 
nothing—it only isolated a certain genotype or genotypes from the origi- 
nal mixture. Tower’s results in selecting for the purpose of creating 
albinic and melanic strains of beetles as illustrated in Fig. 109 may be 
explained in this way. The original population shown at A consisted 
of a number of distinct biotypes. By the isolation of several extreme 
variants Tower separated plus and minus strains which he was able 
to maintain for eight generations by practising intensive selection. In 
the eighth generation he divided each population in half, continuing in- 
tensive selection with one portion and stopping all selection in the other. 
By this method he was able to maintain the plus and minus strains and at 
the same time to observe that in the ninth generation the mode of the 
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