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THE AMEBIC AN NATURALIST [Vol. XLIX 



rats have been reared in the straight selection series, 

 bringing the total number of animals observed in this ex- 

 periment up to 33,249, and the total number of genera- 

 tions of selections up to 17, numbers certainly more nearly 

 justifying the term "mass selection" than those studied 

 by Pearl. As no previous account of this experiment has 

 been given to readers of the Naturalist, a brief review 

 of its salient features may be appropriate here. 



Experiments made by MacCurdy and by Doncaster 

 had shown that the hooded pattern of rats is a Mendelian 

 recessive character dominated in crosses by the "self" 

 or entirely pigmented condition of wild rats and of cer- 

 tain tame races. The F 2 ratio obtained in crosses be- 

 tween hooded and self rats is an unmistakable mono- 

 hybrid ratio, viz., 493 hooded : 1,483 self, or 24.9 per cent, 

 hooded. The hooded pattern is subject to slight fluctua- 

 tions in the relative amounts of pigmented and unpig- 

 mented surfaces, and though these slight plus and minus 

 variations are such as are usually disregarded in Men- 

 delian analyses, MacCurdy 's investigations had indicated 

 that they are to some extent inherited. It was our pur- 

 pose in starting the selection experiments to ascertain 

 whether the observed fluctuations were capable of in- 

 crease and summation through the action of repeated 

 selection, a possibility denied for all such cases by de 

 Vries and Johannsen on theoretical grounds and quite 

 incompatible with notions prevailing then as to the 

 "gametic purity" of recessives. This "pure line" idea 

 Pearl still maintains on the basis of his observations of 

 the winter productiveness of his pullets. But, as I have 

 tried to show, his material is no more adequate than that 

 of Johannsen, which involved no demonstrated Mendelian 

 character whatever. For, though Pearl assumes that 

 winter egg productiveness of fowls involves a "sex- 

 linked Mendelian character" he has withheld from pub- 

 lication the only facts on which such an assumption may 

 legitimately be based. 



Our selection experiments with hooded rats began in 



