riddle: control of sex ratio 325 



eons and that the wider the cross the higher is the proportion of 

 males. Family crosses produce, in nearly all matings, only 

 male offspring. Generic crosses produce from their "stronger" 

 germs — those of spring and early summer — nearly all males. If, 

 however, the birds of such a generic cross be made to " overwork 

 at reproduction," that is if their eggs are taken from them as 

 soon as laid and given to other birds for incubation, then the 

 same parents which in the spring threw all or nearly all male 

 offspring may be made to produce all, or nearly all, female off- 

 spring in late summer and autumn. At the extreme end of the 

 season eggs capable of little, then of no development, are often 

 found in such series. As the parent birds grow older the time of 

 appearance of females, and of eggs incapable of full develop- 

 ment, is reached earlier and earlier in the summer or spring. 



The relation of "width of cross" to the sex ratio in one of the 

 many species (Turtur orientalis) with which he worked is sum- 

 marized 3 in Table 1 . Practically every gradation from the wid- 

 est possible (family) cross to inbreeding shows a sex ratio in 

 accordance with its position in the series. 4 The " family cross" 

 shown in Table 3 has also produced only males. 



In Table 2 I have grouped according to width of cross a num- 

 ber of sex ratios reported by various observers. Here again it 

 is found that family crosses yield only male offspring (20 c? : 9 ) ; 

 generic crosses a ratio of 4.9 d 1 : 1 9 ; specific crosses 4.3 : 1 ; 

 racial crosses 1.9 : 1. The normal sex ratio, i.e., the ratio for 

 any of these species mated to its own kind, is probably nearly 

 1 : 1 or at most not higher than 1.3 d 71 : 1 9 . The method of 

 collecting most of these data renders then objectionable as evi- 

 dence on some important questions, and the numbers are 

 small, but they certainly support the generalization that as the 

 "width of the cross" is increased a relatively higher proportion 



3 The matings included in this table were continued by the present writer; 

 both earlier and later work (to 1914) are included in the summary. 



4 The specific cross — T. turtur and T. orientalis — whose ratio (0.78:1) is a 

 seeming exception is in reality not an exception. One of the females used in 

 this cross had been previously "overworked" and threw nearly all females as a 

 consequence. For complete data see C. O. Whitman, Posthumous Works, Vol. 

 II, chap. 4. The Carnegie Institution of Washington. (In press.) 



