1/2 MAINE AGRICULTURAL EXPERIMENT STATION. I9II. 



1. Probably no line yet obtained is absolutely pure game- 

 tically in respect to fecundity. It represents a mixture of a 

 greater or less number of fecundity genes. 



2. Lines which breed reasonably true to a definite degree of 

 fecundity may in most cases be taken to be made up of indi- 

 viduals bearing a preponderant number of genes of the particu- 

 lar degree of fecundity to which the line breeds true, so that in 

 gametogenesis a great majority of the gametes formed carry 

 only these genes. They also carry some genes of higher, or 

 lower fecundity, or both kinds. When individuals of a definite 

 {e. g., "high") line thus constituted are bred together the ma- 

 jority of the offspring will, purely as a matter of chance, be 

 produced by the union of two high fecundity gametes. It is 

 quite possible that with families of the size obtained with poul- 

 try nearly or quite every individual produced 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 

 "ofif" 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. 163). 



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

 pend 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 practi- 

 cal 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 fecundity. 



.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 particular combination of 

 separately segregable unit characters, can not yet be determined. 

 Every one must recognize the fundamental importance of the 

 investigations of Nilsson-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 



