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 Dichet, furnish definite evidence against contamination of allelo- 
morphs in heterozygous forms. 
CasTLe’s EXPERIMENTS WITH HoopeEp 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, 1914b.) 
“Tt 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, 1916d.) 
Elsewhere (Castle and Phillips, 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 
