42 



THE AMERICAN NATURALIST [Vol. XLIX 



with sister when possible, less often brother with half-sister, 

 rarely cousin with cousin. In this way were obtained 804 young 

 from rigidly selected, closely inbred descendants of a single pair 

 of rats, the series extending into generation F 8 . We have shown 

 (I. c, p. 21) that the progress of selection within this inbred 

 family follows a remarkably close parallel, generation by gen- 

 eration, to the progress of selection in our plus series as a whole. 

 Muller's anticipation that a different result would follow close 

 inbreeding is not justified by our observations. 



In discussing this experiment (p. 21) we have italicized the 

 statement that (so far as the hooded character is concerned) the 

 entire series is derived from a single hooded individual! "When 

 the Hagedoorns made the statement that our stock had not been 

 sufficiently inbred, they had apparently not seen our full pub- 

 lication and so had no means of knowing to what extent it had 

 been inbred, but Muller, with our full publication before him, 

 apparently repeats the statement without taking the trouble to 



W. E. Castle 



October 23, 1914 ' 



NO CROSSING OVER IN THE FEMALE OF THE 

 SILKWORM MOTH 



In a recent review 1 of a paper by Y. Tanaka 2 on linkage in the 

 silkworm moth, I pointed out that some of his data suggested 

 that crossing over was occurring in only one sex. While the data 

 were not sufficient to establish this conclusion, there was at this 

 time another paper by the same author 8 which I had not seen. 

 In this paper are presented data which clear up the matter. 



Tanaka has now made back-cross tests of both sexes. That 

 crossing over does occur in the males was shown by the mating 

 sysy $ X SYsy which gave a total of 865 cross-overs among 

 2,907 offspring. The cross sysy? X SysY^ gave 151/488 as the 

 proportion of cross-overs. But when females were tested, 

 SYsy$ Xsysy^ 1 gave no cross-overs in 1,183 offspring. Tanaka 

 refers to another paper, apparently in press, in which he has 

 shown the same relations (i. e., crossing over in males, none in 



1 Amee. Nat., XLVIII, 1914. 



2 Jour. Coll. Agr. Tohoku Imp. Univ. Sapporo, V, 1913. 

 a Jour. Coll. Agr. Tohoku Imp. Univ. Sapporo, VI, 1914. 



