194 BIOLOGY OF DEATH 



l x curve, but the maximum degree of longevity attained 

 is much smaller, amounting to about half of that attained 

 in the other four lines. Both lines 61 and 64 tend to show 

 in general a curve convex to the base, especially in the 

 latter half of their course. 



Since each of these lines of descent continues to show 

 through successive generations, for an indefinite time, 

 the same types of mortality curves and approximately 

 the same average durations of life, it may safely be con- 

 cluded that there are well marked hereditary differences 

 in different strains of the same species of Drosophila in 

 respect of duration of life. Passing from the top to the 

 bottom of the diagram the average expectation of life is 

 reduced by about two-thirds in these representative 

 curves. For purposes of experimentation, each one of 

 these lines of descent becomes comparable to a chemical 

 reagent. They have standard durations of life, each 

 peculiar to its own line and determined by the hereditary 

 constitution of the individual in respect of this charac- 

 ter. We may, with entire justification, speak of the 

 flies of line 64 as hereditarily short-lived, and those of 

 line 55 as hereditarily long-lived. 



Having established so much, the next step in the analy- 

 sis of the mode of inheritance of this character is ob- 

 viously to perform a Mendelian experiment by crossing 

 an hereditarily short-lived line with an hereditarily long- 

 lived line, and follow through in the progeny of succes- 

 sive generations the duration of life. If the character 

 follows the ordinary course of Mendelian inheritance, we 

 should expect to get in the second offspring generation a 

 segregation of different types of flies in respect of their 

 duration of life. 



Figure 49 shows the result of such Mendelian experi- 



