598 



THE AMERICAN NATURALIST [Vol. XLIX 



spring individual's parentage, both male and fe- 

 male, was known. 



5. The records of production of the progeny of each 

 mating separately recorded and studied as a unit. 



It will be noted that there are but two essential differ- 

 ences between the plan in this period and that followed 

 in the earlier one. These are : first and most important, 

 that in this second period the principle of progeny testing 

 was introduced into the scheme of breeding. The second 

 difference was that selection was carried on for low pro- 

 duction as well as for high, which had not been previously 

 done. A third difference is apparently found in the fact 

 that in this second period of selection the winter record 

 rather than the yearly record is made the basis of selec- 

 tion. This is in no way an essential difference. The 

 reasons for adopting the winter period have been set 

 forth in detail elsewhere 4 and need not be repeated. It 

 suffices to say here that essentially the same results and 

 conclusions will be reached if one uses winter production 

 or annual production. 



As a result of the studies made in this period on the 

 plan of breeding outlined the mode of inheritance of the 

 character winter production was definitely determined, 

 and has been confirmed by subsequent work. 5 The char- 

 acter was shown to be Mendelian in its genetic behavior, 

 depending upon two factors, one of which is sex-linked. 



3. The Period from 1912 to Date.— The only difference 

 in the mode of breeding the stock of Barred Plymouth 

 Eocks in this period, as compared with the preceding one, 

 is found in the fact that during this last period all selec- 

 tions for low and mediocre production hare been dropped. 

 The breeding for high production alone continues, with 

 only such differences in the details of manipulation of 

 the breeding stock as would naturally follow a definite 

 knowledge of the mode of inheritance of the character, 



« Pearl, 1912, loc. cit., and Me. Agr. Exp. Stat. Ann. Kept., 1914, pp. 217- 

 236. Cf. also Wilson ami Murphy. Jour. Dept. A,,r. Inland, Vol. XIV, 



• Pearl, 1912. loc. cit., also Amer. Nat., Vol'. XLTX, 1915. pp. 306-317, 

 an,l Curtis and Pearl, Jour. Exp. Zoology, Vol. 19, 191.5. pp. 45-59. 



