222 BIOLOGY OF DEATH 



produced by administration of tethelin are of a wholly 

 different and smaller order than those which have been 

 shown, in the earlier portion of this chapter, to exist be- 

 tween pure strains of Drosophila which are based upon 

 hereditary differences. 



Putting together all the results which have been re- 

 viewed in this and the preceding chapter, it appears to 

 be clearly and firmly established that inheritance is the 

 factor of prime importance in determining the normal, 

 natural duration of life. In comparison with this factor, 

 the influence of environmental forces (of sub-lethal im- 

 mediate intensity of course) appears in general to be 

 less marked. 



