No. 592] SHORTER ARTICLES AND DISCUSSION 251 



such form that any other interpretation of the data could by any 

 chance be tested. ' ' If this statement is true, it is because of my 

 inability to devise any other form in which to present the data. 

 I have presented it in such form that the limits adopted for the 

 categories of variation could be shifted at will and I am ready to 

 be shown how its presentation can be further improved and sim- 

 plified. Pearl suggests that my omission pertains to the individ- 

 ual pedigree of the rats, in which suggestion he echoes a thought 

 of the Hagedoorns on which I have twice commented elsewhere, 

 showing, I think, that the alleged defect does not exist, for the fol- 

 lowing reasons : 



1. It is impossible for a colony of 33,000 rats to be produced 

 from an original stock of less than a dozen animals, with con- 

 stant breeding together of those which are alike in appearance 

 and pedigree, and with continuous selection of extremes in two 

 opposite directions, without the production of pedigrees which 

 in the course of each selection experiment interlock generation 

 after generation and finally become in large part identical with 

 each other. This has been repeatedly verified in individual eas"s, 

 but is incapable of a more generalized statement or of demonstra- 

 tion in generalized form. At least I am unable to devise such 

 demonstration. 



2. In a specific case described on pp. 20 and 21 (Castle and 

 Phillips) a selection experiment was started with the hooded F 2 

 offspring of a single selected hooded and a single wild rat and 

 this experiment was carried through the F 8 generation leading to 

 the production of 804 young from rigidly selected, closely inbred 

 descendants of a single pair. We showed (1. c, p. 21) that the 

 progress of selection within this inbred family follows a remark- 

 ably close parallel, generation by generation, to the progress of 

 selection in our plus series as a whole. Here there can be no 

 question of a difference in pedigree among the selected animals. 

 This is eliminated as a possible factor in the result. Can Pearl 

 suggest any other possible factors capable of elimination? If so, 

 I should be pleased to give attention to them. 



I humbly beg pardon for having made the all too obvious sug- 

 gestion that environmental conditions, and in particular size of 

 flock, may affect average flock fecundity. And yet I find that 

 Pearl himself elsewhere lays great stress on this point. My chief 

 offense seems to lie in my failure to realize that he had already 

 taken all possible precautions in this matter, and that he consid- 



