SIZE INHERITANCE IN DUCKS 145 



of weight in F2 over their variability in Fi and the females show 

 a very small increase. The numbers are now thought large 

 enough to have a significance. 



2. If a stock like the pure wild race mentioned above, could 

 be used as a small parent, it is very likely that a much more 

 striking segregation of size might be observed. 



3. The present experiment does not throw any further light on 

 the theoretical side of the question except to diminish the possi- 

 bility of the existence of any large and clear cut size units in birds, 

 which would result in an easily recognized numerical ratio. 



4. The male sex is much more variable in size than the female. 



5. The Rouen race used in this work was homozygous for color 

 (mallard color) when crossed with a mallard race. 



6. The growth curves show that a very satisfactory age for 

 studying size of ducks is 140 to 150 days, at the assumption of 

 the first adult plumage, but also point to a marked effect of earli- 

 ness and lateness of hatch on the rate of growth and ultimate size. 

 This difference probably varies greatly from year to year and 

 cannot be regarded as uniform. 



7. A disturbed sex ratio occurs among the F 1 hybrids, a result 

 apparently of the difference in size of the parents, for it is not 

 seen in Fo. This ratio has resulted in the preponderence of males 

 over females in the proportions of nearly two to one. 



BIBLIOGRAPHY 



GooDALE, H. D. 1911 Studies on hybrid ducks. Jour. Exp. Zool., vol. 10, p. 241. 

 GoLDSCHMiDT, R. 1913 Zuchtversuche mit Enten. Zeit. fur Induct. Abstamm- 



ungs- und Vererbungslehre. Bd. 9, p. 161. 

 GuYER, M. F. 1909 On the sex of hybrid birds. Biol. Bui., vol. 16, p. 193. 

 Phillips, J. C. 1911 Inheritance of size in ducks. Jour. Exp. Zool., v. 12, p. 369. 

 Smith, Geofpry, and Mrs. Haig-Thomas. 1913 On sterile and hybrid pheasants. 



Jour. Genetics, vol. 3, p. 39. 



