The conclusions^ drawn from these and other results 

 are that variations in the weight, length and breadth of 

 seeds within a Pure Line are not heritable. As DeVries 

 says, "A Pure Line is completely constant and extremely 

 variable," that is, germ plasm is constant but fluctuating 

 variations are common. 



Discuss the value of Pure Line conception in hered- 

 ity, and how does it harmonize with Galton's laws of 

 Inheritance and Darwin's theory of Natural Selection? 



Why is continuous selection within a variety ne- 

 cessary in some crops and not in others? 



Give examples of Pure Lines in Nature. 



Selection of Hooded Rats. — 



Castle^ and Phillips carried out a valuable series of 

 selection experiments with a colony of hooded rats where 

 the amount of pigment in the hood and the dorsal band 

 varied. Two series of selections were made for 16 genera- 

 tions, one, a minus, toward a lighter type and the other, a 

 plus, toward the darker type. In both series steady pro- 

 gress was made towards light in one case and dark in 

 the other. Castle concluded from the above results that 



Fig. 13. — Scheme to show classes of hooded rats 

 used by Castle. (After Castle.). 



inasmuch as the hooded character is a clear example of a 

 sharply segregating unit-character, a unit-character "is 

 subject constantly to slight quantitative variations which 

 are themselves to some extent hereditary." Moreover, he 

 says "the changes affected by selection show permanency 

 under crosses with wild races," behaving as a simple re- 

 cessive unit. (Fig. 13). 



(1) — See Castle's "Genetics and Eugenics" for a criticism of the Pure 

 Line theory. 



(1) — Dr W E. Castle, Professor of Zoology. Harvard University, has 

 made valuable contributions to our knowledge of invertebrates, 

 and to the more intricate aspects of heredity and gentics. 



73 



