LAW OF VARIATION 235 



gametic constitution of the male is perceptible in all the progeny, 

 while that of the female appears in a smaller percentage of cases. 



2. That it is possible systematically to breed males with a high 

 productive gametic constitution. 



3. That in breeding to increase egg laying the production has 

 been divided into two kinds, namely, (a) normal or natural produc- 

 tion, which takes place during the natural breeding season or spring 

 months, the average in this period being about thirty eggs; and (6) 

 excess laying period, which is possessed by fewer birds, and which 

 represents from 100 to 175 eggs laid during the fall and winter. The 

 problem, then, is how to breed birds which shall possess in their 

 gametic make-up germ cells representing both of these periods. 



4. That improvement in egg production comes about by raising 

 the general average through elimination of the poor producers; 

 or, in other words, by bringing the mass to a higher level, and not 

 to any great extent by raising the standard and improving the best. 



5. That in order to accomplish this improvement, as above 

 outlined, it is necessary to know what has been the individual 

 performance of the members of the flock for a number of genera- 

 tions in succession, and this can only be determined by keeping 

 small breeding pens and by trap nesting. 



While the application of the genotype theory is comparatively 

 new, studies so far are in entire accord with it. They indicate, 

 first, that fecundity in fowls is transmitted; second, that this 

 inheritance is in accord with the genotype concept, even where it 

 is almost impossible to establish true blood lines. 



Law of Variation. This law may be defined as the tendency of 

 individuals to produce progeny which differ in type from either par- 

 ent. It is constantly working in opposition to the law of heredity, 

 and might be expressed as the law that " like does not produce like." 



In the light of present knowledge, the causes of variations are 

 in many cases obscure. It will be noted, however that in many 

 cases the progeny are not like the parent. In some instances the 

 differences may be slight and exhibited only in one or two charac- 

 ters, while in others the variations may be very marked and cover 

 a great variety of traits and forms. As applied to poultry two 

 general principles are held: 



1. That all characters, whether structural or functional, are 

 exceedingly variable. 



2. That ordinary variation is the result of a change in the rela- 

 tions between a number of closely associated characters brought 



