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THE AMERICAN NATURALIST [Vol. XLV 



in the line for several successive generations may be of 

 this kind. In the long run, however, it is to be expected 

 that a small number of ''off" individuals will appear 

 in the line. These originate by the chance union of two 

 low fecundity genes, or by the union of a "high" gene 

 with a "low" gene of great potency (as in the case of 

 D31, cf. p. 335). 



3. The degree to which such a line will breed true will 

 depend upon the proportion of genes of one type (or of 

 very similar types) present. The higher such proportion 

 the less frequently will the "off" individual segregate 

 out. The practical goal to be worked towards is, of 

 course, to obtain several lines not closely related, but all 

 made up only of individuals homozygous with respect to 

 either high or low or any other definite degree of fecun- 

 dity. 



Whether a given degree of fecundity is to be regarded 

 as a single unit character, in the Mendelian sense, or, on 

 the other hand, as a complex dependent upon a particu- 

 lar combination of separately segregable unit characters, 

 can not yet be determined. Every one must recognize 

 the fundamental importance of the investigations of 

 Xilsson-Ehle, Baur and East, which have shown that 

 many characters which at first glance do not appear to 

 conform to any determinate law of inheritance are really 

 complexes, formed by the combination of a number of 

 unit characters, each of which segregates and otherwise 

 behaves in a perfectly regular and lawful manner. 

 There are some facts which indicate that high fecundity 

 is a character of this kind, but it will require prolonged 

 analysis to decide this, because of the numerous practical 

 difficulties which attend the study of fecundity. 



A great help in this analysis, as well as a contributory 

 line of evidence of much weight in supporting the general 

 conception of the manner of inheritance of fecundity set 

 forth above, is derived from the study of crosses be- 

 tween breeds of poultry in which high and low degrees 

 of fecundity are definite breed characters. Studies of 



