PURE LINES 259 
Significance of the Pure Line Theory in Breeding.—The que tion 
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 “Normal” Range of Variation 
‘a Mode my Mode 
= —— 
5 
4 
Vv fax 
aii XI 
IV LEN a 
Q | 
§ soe ed x 
S [4 Ni 
5 0 
o FE = | 
o Z= == 
IX 
EY AS 
Ul ‘ 
\ IN 
— ——— = = VIII 
I a | Z = 
( = [EN wi 
VII == f S vil Z 
. yz 8 
< = — r i 
aN = 5 
VI ==) z i+4VI | oO 
a aN 
a . 
a 
§ z =e 
» a 
: ==0y 
5 IV ri AGA 
0 
Was 
III = 7 
B z=N A 
II P= \/ 
| Peal J 
Fic. 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 
Digitized by Microsoft® 
