THE PURE LINE AND SELECTION 145 



These four bristles are ordinarily strictly accounted 

 for in heredity but the occasional variation in their 

 number led MacDowell, and later others, to attempt to 

 establish by selection a new style in these bristly 

 decorations consisting of either extra 

 or fewer bristles. Apparent success 

 was the result in effective selection 

 among the offspring of parents 

 homozygous for the single character 

 of four bristles. 



c. Pearl's Egg-laying Hens 



In an experiment extending over 

 17 years and which involved nearly 

 5000 pedigreed birds, Pearl tried, 

 within a homozygous strain, to select biological Cin- 

 a hen that would produce 00 eggs Drawn from 

 annually instead of the ordinary Bridges Fi ^ C ' J * 

 number of 125, but without success. 



d. Castle's Hooded Rats 



Finally one of the most famous selection experiments 

 on record is that of extent of pigmentation, plus and 

 minus, in the hooded rat. This experiment involved 

 breeding an average of nearly twelve rats a day without 

 cessation for eight years and it has not only made the 

 Pied Piper of Hamelin roll over in his grave but has 

 kept biologists busy with explanations of the results, 

 for, like the four bristles on Drosophila's back, it ap- 



