AN ANALYSIS OF THE EFFECT OF SELECTION. 47 



the normal than were those of curved and of balloon flies that had been 

 kept in pure stocks. These results, taken in connection with the data 

 presented above for bristle number in flies from lines heterozygous 

 for Dichaet, furnish definite evidence against contamination of allelo- 

 morphs in heterozygous forms. 



Castle's Experiments with Hooded Rats. 



Perhaps the best known selection experiment is that carried out by 

 Castle and various collaborators (Castle and Phillips, 1914, Castle 

 and Wright, 1916, etc.) with hooded rats. The theoretical conclu- 

 sions reached by Castle are not in agreement with those arrived at 

 by various other investigators, including the author, although for the 

 most part the data obtained are very similar. Castle's results have 

 been discussed by Muller (1914a) and MacDowell (1916), who have 

 shown in detail that all the data known to them were explainable on 

 the multiple-factor view, without recourse to such hypotheses as 

 contamination of factors or production of factorial variations by selec- 

 tion. One point has, I think, not been sufficiently emphasized by 

 them, namely, that the rat experiments are hard to evaluate properly 

 until we are in possession of more accurate data regarding the pedi- 

 grees. Since these two criticisms were written, Castle (Castle and 

 Wright, 1916) has given some additional data, which he has used, 

 in a reply (Castle, 1917) to MacDowell's paper, as arguments against 

 the latter' s conclusions. 



With regard to the question of pedigrees, to take up these ques- 

 tions in order, the main point on which information is desired is: 

 How closely inbred were the rats, both before and after the beginning 

 of the selection experiment? The following quotations contain most 

 of the available evidence on this matter: 



"Since the entire stock is descended from a very few individuals (less than 

 a dozen), and we have at no time hesitated to mate together brother and 

 sister, provided they varied in the same direction, but have always used the 

 most extreme individuals (plus or minus) which were available, to mate 

 with each other, it follows that very close inbreeding must have occurred 

 throughout the experiment." (Castle, 19146.) 



"It is impossible for a colony of 33,000 rats to be produced from an original 

 stock of less than a dozen animals, with constant breeding together of these 

 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 cases, but is incapable of a more 

 generalized statement or of demonstration in generalized form. At least I 

 am unable to devise such demonstration." (Castle, 191Qd.) 



Elsewhere (Castle and PhilUps, 1914, p. 20) it is stated that part 

 of the original stock consisted in a mixed lot of trapped rats that "had 

 probably arisen by the crossing of an escaped albino rat with wild 



