404 THE AMERICAN NATURALIST [Vol. LVI 



2. When compared exactly, on the basis of comparable 

 life spans, the human being has at every equivalent age 

 a higher relative expectation of life than does Droso- 

 phila, measured in terms of its own life span in each 

 case. That this was the case for all but old age was 

 concluded from the rough graphical comparisons of the 

 first Study in this series. It is now seen that the same 

 is true over the whole of life. From this fact the con- 

 clusion appears warranted that while the laws of mor- 

 tality are fundamentally the same in hind for Drosophila 

 and for man, they differ somewhat quantitatively. There 

 is a temptation to conclude further that the quantitative 

 difference finds its cause .in man's own control and 

 amelioration of his environment though sanitation and 

 hygiene. Such a conclusion, however, seems to me not to 

 be strictly warranted, in the light of our present knowl- 

 edge. There is some suggestion that it is true, as was 

 pointed out in the first of these Studies, from the fact 

 that the progressive change of the human h curve in 

 form during historical times has been in the direction 

 of moving from the form typical of Drosophila to that 

 now found for progressive, highly civilized groups of 

 men. But definitive conclusions on the point must await 

 further research. 



3. The details of the quantitative differences in the 

 two curves are interesting. When the first 25 per cent, 

 of the equivalent life spans has been passed Drosophila 

 has lost almost exactly 25 per cent, of the individuals 

 starting life together, while man has lost but 10 per cent. 

 When 50 per cent, of the life spans has been completed 

 Drosophila has lost 72 per cent, of the individuals start- 

 ing together, while man has lost but 31.5 per cent. At 

 75 per cent, of the life span, Drosophila has lost 94 per 

 cent, of the individuals and man 77 per cent. From the 

 53d centile of the equivalent life spans on practically 

 to the end, man has more than tw^ce as many survivors 

 out of a thousand starting together as does Drosophila. 



Exactly similar results to those here presented are ob- 



