R 0. PUNNETT AND P. Gr. BaILBY 25 



if not before, and the weight reached by them at the age of 35 weeks is 

 not exceeded for several months. Later on the bird may show a further 

 period of growth, but after it has reached the age of 8 — 9 months, or in 

 many cases even less, there is a distinct interval of several months 

 during which the weight remains nearly constant. This is certainly 

 true for the breeds with which we have been working, and we have 

 consequently kept and recorded the weights of the birds bred up to 

 35 weeks, after which the majority have been killed. When we speak 

 of the weight of a bird we mean this weight of the first year's gi'owth 

 unless the contrary is stated. We may add that all our birds were kept 

 together on the same land and were fed and treated, so far as possible, 

 precisely in the same way. 



The Hamhwrgh-Sebright Gross. 



The original stock with which the experiments were started in 1910 

 consisted of a trio of Gold-pencilled Hamburghs and 2 Silver Sebright 

 hens. The Hamburgh (/ when 3 years old weighed 1540 grams; when 

 35 weeks old it is likely that he would have been some 150 — 200 gxams 

 lighter. The Sebright $ $ weighed 570 and 620 gi-ams respectively. 

 We have no record of the weight of a Silver Sebright cock of this 

 strain but he would probably have weighed about 750 grams. During 

 the course of our experiments we also used some Gold Sebrights. 

 These were procured from a different source and were markedly smaller 

 than the Silvers, a cock weighing but 570 grams and 3 hens 455, 455, 

 and 480 grams respectively. We raised a few birds from our original 

 Hamburgh (/ with one of the Hamburgh hens, viz. 1 ^ and 4 $ ^ . In 

 both sexes these birds were rather larger than the F^ ex Sebright x 

 Hamburgh (cf Table I, p. 26). We subsequently mated up a brother 

 and sister from these pure Hamburghs. The offspring were distinctly 

 smaller and throughout their growth were sickly looking birds. Whether 

 due to close inbreeding or to some other cause the phenomenon was 

 very marked, and we have not made use of observations on these birds 

 in our account. With healthy birds the weight of the Hamburgh is 

 nearly double that of the Sebright. 



The original cross was made between the Silver Sebright hen and 

 the Hamburgh cock (cf. PI. IV, fig. 1). From it two cockerels were 

 reared in 1910, and in the following year 8 (/J" and 7 $ ^ were raised. 

 With regard to size both sexes were fairly uniform. At 35 weeks old 

 the (/'(/' ranged from 1140 to 1360 grams, while at the same age the 

 $ % were between 940 and 1110 grams (cf Table I). 



