316 



THE AMEBIC AN NATURALIST [ Vol. LII 



in Pearl's Barred Plymouth Hocks. However, the re- 

 sults obtained by Dryden at the Oregon Station show 

 that individual selection in pedigreed lines as opposed to 

 mass selection may result in improved egg production 

 quite as well as by the application of Pearl's theory. 



Egg production in the domestic fowl may seem at first 

 sight to be a highly desirable character on which to 

 study the influence of selection. It may be regarded as a 

 unit character if one so desires, and if, by selection, this 

 character is changed, it is clear that selection has been 

 effective. But it is also clear that the effectiveness of 

 such selection in this instance rests, in large measure, at 

 least, upon the influence exerted by various modifying 

 factors, such as broodiness or age at first egg, discussed 

 in this paper. It is possible to study these factors indi- 

 vidually both by themselves and also in their relation to 

 egg production. Broodiness is known to behave like a 

 Mendelian dominant, while Pearl has shown that the rate 

 of production during the winter cycle is dependent on two 

 genes, one gex-linked, the other a simple Mendelian char- 

 acter. We have found some evidence that the presence 

 or absence of a winter cycle in fowls that lay at all during 

 the winter 7 follows the Mendelian scheme. Since the 

 influence of the various modifying factors is so clear cut, 

 it is evident that egg production is a character wholly 

 unsuited for studying the possibility of the modification 

 of the germinal representatives of a character by selec- 

 tion. On the other hand, it is a good example of a char- 

 acter that varies continuously, but the continuity of whose 

 variability can be shown to depend upon several modify- 

 ing factors. 



There is a point of some general interest regarding the 

 genetic composition of any given flock. Hardy ('08) 

 showed that the proportions in which a Mendelian char- 

 acter occurs tend to remain constant provided no selec- 

 tion is practised. Fanciers often practise a certain but 



out tbe winter and spring, and not absence of egg ] 

 i Barred Plymouth Rocks. 



