STUDIES ON THE DURATION OF LIFE 193 



the survival distribution of the offspring of a single 

 brother and sister pair mated together. In forming a 

 line a brother and sister are taken as the initial start 

 because by so doing the amount of genetic variation pres- 

 ent in the line at the beginning is reduced to the lowest 

 possible minimum. It should be said that in all of the 

 curves in Figure 48, both male and female offspring are 

 lumped together. This is justifiable for illustrative pur- 

 poses because of the small difference in the expectation 

 of life at any age between the sexes. The line of descent, 

 No. 55, figured at the top of the diagram, gives an l x 

 line extraordinarily like that for man, with the exception 

 of the omission of the sharp drop due to infantile mor- 

 tality at the beginning of the curve. The extreme dura- 

 tion of life in this line was 81 days, reached by a female 

 fly. The l x line drops off very slowly until age 36 days. 

 From that time on, the descent is more rapid until 72 days 

 of age are reached when it slows up again. Lines 50, 60, 

 and 58 show l x curves all descending more rapidly in the 

 early part of the life cycle than that for line 55, although 

 the maximum degree of longevity attained is about the 

 same in all of the four first curves. The general shape 

 of the lx curves changes however, as is clearly seen if 

 we contrast line 55 with line 58. The former is concave 

 to the base through nearly the whole of its course, whereas 

 the l x curve for line 58 is convex to the base practically 

 throughout its course. While, as is clear from the dia- 

 gram, the maximum longevity attained is about the same 

 for all of these upper four lines, it is equally obvious 

 that the mean duration of life exhibited by the lines falls 

 off as we go down the diagram. The same process, which 

 is in operation between lines 55 and 58, is continued in 

 an even more marked degree in lines 61 and 64. Here 

 not only is the descent more rapid in the early part of the 



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