392 THE AMERICAN NATURALIST [Vol. LVI 



bottles. Another consideration is that genetically Quin- 

 tuple carries factors for very short life. These genes 

 appear, in our experience with these flies, to be the over- 

 whelmingly important factors in determining their 

 length of life. No environmental factor, however favor- 

 able, makes much difference in their duration of life. 



Summary 



In experiments involving the deteraiination of the 

 duration of life in 2,576 individual flies, it has been 

 found that in the case of DrosopJiila of wild type {i.e., 

 carrying no mutations so far as known), an increase of 

 roughly 10 per cent, in the mean duration of life is 

 brought about by increasing the ventilation of the cul- 

 ture bottles, by covering the mouth with one layer of 

 No. 48 mesh bolting cloth, as compared with the use of 

 cotton plug stoppers as is the usual practice in the cul- 

 ture of Drosophila in the laboratory. Owing, in our 

 opinion, to fundamental differences in behavior, no such 

 difference appears in the case of Quintuple flies. 



B. Can the Duration of Life be Increased by 

 Embryonic Juice? 

 If the theory of senescence and natural death which the 

 senior author has developed in his ''Biology of Death" 

 (1-7) is true, one consequence of it should be that it 

 might be possible to increase the duration of life, if by 

 appropriate means one could restore the normal func- 

 tional balance of the parts of the body after changes had 

 set in with advancing age. Might it not be possible, by 

 the use of X-rays for example, at the right stage of the 

 life curve, and in proper dosage, to destroy cells, or per- 

 haps even parts of tissues, which have got out of proper 

 functional balance, and thus pave the way to their re- 

 placement by regeneration with fresh, ''young" cells or 

 tissues? In this way the life of the whole organism 

 might be prolonged. The work of Frisch and Starlinger 

 (45) with blood suggests that such a result might at least 



