I/O MAINE AGRICULTURAL EXPERIMENT STATION. I9II. 



is much overlapping of individuals in the different lines. In 

 consequence it results that the egg record of an individual bird 

 is of almost no value in helping to tell in advance of the breed- 

 ing test to what fecundity genotype it belongs. Essentially this 

 same fact has been brought out in all of the work which has 

 been done with pure lines. The only difference in the present 

 case lies in the fact that the range and degree of variation with- 

 in the line appears to be relatively greater in the case of fecun- 

 dity than in the case of most characters hitherto studied, as, for 

 example, size relations in beans or Paramecium. 



The most serious difficulty which confronts one m the attempt 

 to analyze the inheritance of a character like fecundity lies in 

 the almost inextricable mingling of genotypes in the great ma- 

 jority of individuals. This, of course, is a direct consequence 

 of the manner of reproduction. The germ plasm of two sepa- 

 rate individuals must unite to form a new individual. By pro- 

 longing incestuous mating one may in theory come indefinitely 

 close to reproductive purity, but in practice even this is ex- . 

 tremely difficult, if not impossible of accomplishment on any 

 large scale or through any long period of time. The fact simply 

 is that a "pure line" in the strict sense of Johannsen * can not 

 by definition exist in an organism reproducing as the domestic 

 fowl does. This, however, by no' means indicates that the in- 

 heritance of fecundity does not rest on a genotype basis, or, in 

 other words, that fowls do not carry definite genes for definite 

 degrees of fecundity. 



We touch here upon an important point ; namely, the relation 

 of the mode of reproduction to the mode of inheritance. As 

 one reflects upon the matter it becomes clear that it is only in 

 the sense of a reproductive line that we can not, by definition, 

 have pure lines in organisms where the sexes are separate. It 

 is perfectly possible to have a line of such organisms in which 

 all the individuals are gauietically pure with reference to any 

 particular character. For example, it is the simplest of matters 

 to establish a line of horses pure in respect to chestnut coat 

 color. Any individual in such a line mated to any other will 



* Johannsen's definition is as follows : "Mit einer reinen Linie bezeichne 

 ich Individuen, welche von einen einzelnen selbstbefruchtenden Indi- 

 viduum abstammen." ("Ueber Erblichkeit in Populationen und reinen 

 Linien," p. 9.) 



