20 PIEBALD RATS AND SELECTION. 



away with it? We do not think so, but to this question we shall return 

 again. 



The question might be asked whether the modifications produced in 

 the selected races by a cross with wild or Irish stock are likely to be 

 more or less permanent than those produced in unselected races by the 

 same means. A single experiment was made which bears on this 

 question in relation to the Irish cross. One of the — 2.00 grandchildren 

 recorded in the third row of Table 43 was mated with —2.00 individuals 

 of the uncrossed stock of the minus series and produced nine young of 

 mean grade —0.63, the expectation for the uncrossed race of the same 

 grade and generation being about —1.90. In other words, this ex- 

 tracted —2.00 individual regressed (in breeding) as if it really had been 

 affected by the cross, even though it did not show it, but the number of 

 young is so small that no emphasis should be placed upon this result. 



From the experiment recorded in the last row of Table 43 were 

 obtained extracted individuals of mean grade +1.37, which as parents 

 produced 16 young of mean grade +1.68, or, in other words, offspring 

 about like themselves. Hence the changes effected by a cross are per- 

 manent, like those effected by selection. 



PLUS SELECTION OF "EXTRACTED HOODED" RATS. 



It has been suggested that the original material out of which the 

 plus series came consisted of modified individuals produced by a cross 

 with the wild race. This was not known positively to be so, because 

 part of the original stock (with which MacCurdy worked) consisted of 

 hooded black and hooded gray rats captured in company with gray self 

 and black Irish rats and albinos. Subsequent experiments showed that 

 ordinary albino rats, if crossed with wild gray ones, will produce in F2 

 all these classes of individuals. This indicated pretty clearly that the 

 particular colony which had fallen into our hands had probably arisen 

 by the crossing of an escaped albino rat with wild ones. But it still 

 remained uncertain what sort of hooded pattern the escaped albino had 

 transmitted and whether or not this had been influenced by the wild 

 cross. We therefore determined to ascertain whether out of our minus 

 series crossed with wild a plus series could be derived. To this end 

 certain of the F2 extracted hooded individuals (entered as grandchildren 

 in Table 42, row 1, and descended from a single hooded individual of 

 grade —1.87, generation 2^) were mated inter se, thus producing an F3 

 generation. Table 44, second row. The selected individuals were the 

 aberrant male of grade +3.50 and females of grade +1.50, so that the 

 mean grade of the chosen parents (extracted from the crossed minus 

 series) was +2.50. They had 34 young ranging in grade from to 

 +3.50, mean +2.06, a regression of 0.44 toward 0, repeating the phe- 

 nomenon regularly found in both selection series. 



In this same experiment some F2 parents of mean grade —0.75 had 

 19 young (first row of Table 44), whose mean grade was -0.04, a 



