STUDIES ON THE DURATION OF LIFE 195 



ment performed on a large scale. In the second line from 

 the top of the diagram, labeled ' 1 Type I l x , ' ' we see the 

 mortality curve for an hereditarily long-lived pure strain 

 of individuals. At the bottom of the diagram the ' ' Type IV 

 l x ' ' line gives the mortality curve for one of our heredita- 

 rily short-lived strains. Individuals of Type I andTypelV 



1.000 



900 



I 



0O 



AGL IN DAYS 



FIG. 49. Life lines showing the result of Mendelian experiments on the duration of life in 



Drosophila. Explanation in text. 



were mated together. The result in the first offspring 

 hybrid generation is shown by the line at the top of dia- 

 gram marked * l F x l x . " The F x denotes that this is the mor- 

 tality curve of the first filial generation from the cross. 

 It is at once obvious that these first generation hybrids 

 have a greater expectation of life at practically all ages 

 than do either of the parent strains mated together to 

 produce the hybrids. The result is exactly comparable 

 to that which has for some time been known to occur 

 in plants, from the researches particularly of East and 

 others with maize. East and his students have worked 



