238 



Biology in America 



But the voice of the selectionist is not yet stilled. Perhaps 

 the chief American exponent of the theory has been Professor 

 Castle of Harvard, whose work on hooded rats is well known 

 to all biologists. The hooded rat is a color variety of the 

 common brown rat, with black head and shoulders and a black 

 stripe on the middle of back and tail. By persistent selec- 

 tion of individuals with more black and more white respec- 

 tively, Castle has obtained individuals ranging all the way 

 from those with a small median stripe of white upon the belly 

 to others with only a small patch of black upon the snout. 3 



For over twenty years a selection experiment has been 

 conducted at the experiment station at the University of Il- 

 linois, which also appears to give strong support to the the- 



" HOODED " EATS 



Showing the results of experiments by Professor Castle of Harvard in 

 selecting these animals for extent of black and white in the pattern. 

 After Castle & Phillips, "Piebald Eats and Selection/ ' Carnegie In- 

 stitution, Publication No. 195. 



ory. In 1896, Professors Hopkins and Smith began selecting 

 seed from ears of corn which had on the one hand a high, and 

 on the other a low protein content. This continual selection 

 has resulted not only in producing grades of high and low 

 content, but each succeeding year shows on the average a wider 

 divergence in the two grades. Thus, starting in 1896 with 

 an average per cent (in weight of grain) of 10.92 they pro- 

 duced the first year a high grade of 11.10% and a low of 

 10.55% with a difference of only 0.55%. By gradually in- 

 creasing steps these figures after twenty years' selection 

 reached 14.53% for the high, 7.26% for the low and 7.27% 

 for the difference. Similar results were obtained by them in 

 selecting for oil content. 



The anti-selectionists however have explained these results 

 as due merely to the sorting out of variations already pres- 

 ent in a very mixed stock. A better understanding of the 



Eecently Professor Castle has modified his original contention. 



