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TRE AMERICAN NATURALIST [Vol. LI 



while ewes from three to six years of age seemed to be at 

 the optimum breeding period of their life. Humphrey 

 and Kleinheinz (6) found that two-year-old ewes pro- 

 duced 141 per cent, of lambs and six-year-olds, 191 per 

 cent. Possibly the writers do not understand the tables 

 presented by them, but their calculations on the basis of 

 the data there given would show the following averages at 

 each age : 



TABLE YII 



Pearl (9) made a biometric study of the fertility of a 

 long-lived ewe whose breeding record was as follows: 



TABLE VIII 



Assuming that the ewe was about one year old when the 

 first lamb recorded was born, Pearl found that the mean 

 point of the ewe's effective breeding life was 8.57 years, 

 that the median point was 8.17 years and that the modal 

 breeding point, or the point of maximum fertility per unit 

 of time, was at 7.34 years. 



Taking into account the seventeen years in which some 

 young were born, the following constants regarding the 

 number of lambs per birth were found : 



