352 Harry Auerbach 



above equation quite closely for almost the entire life span, excluding only the first few months. 

 On the other hand the DBFi hybrids (dilute brown female x C57 male) exhibit the Gompertz 

 function as may be seen in Fig. 5 of that paper. 



The dilute brown strain is characterized by a high rate of mammary cancer, while the hybrid 

 has a low rate. The Marsh albino is another high-cancer-rate strain, which, although it does 

 not have a survival curve of the form log ///o = —a.X^, does, when crossed with the C57, produce 

 hybrids with a much longer life span. The survival curve is of the Gompvertz type. Changes in 

 the genetic characteristics associated with hybridization do not just change the constants of an 

 equation of the Gompertz form, but rather the survivorship curve is of a different form. 



