PURE LINES 



259 



Significance of the Pure Line Theory in Breeding. The question 

 thus arises: How does the pure line theory explain the fact that man has 

 wrought profound changes in domesticated animals and plants by selec- 



" Normal" Range of Variation 

 ' Mode 



IV 

 III 



II 



I 

 VII 



VI 



IV 



in 



/\ 



'Normal" Range of Variation 



/\ 



A 



T 



XI 



IX 



VIII 



VII 



VI 



IV 



III 



FIQ. 107. Diagrammatic representation of results of three experiments in selecting 

 beetles in an effort to create an albinic strain from a pure strain. (From Tower.) 



tion? It is well known that as a rule a mixed population consists of a 

 number (probably quite large) of distinct biotypes and that in autoga- 

 mous species these biotypes are pure lines to begin with, while in alloga- 

 mous species it is only by continued intensive selection that existing 



