190 BIOLOGY OF DEATH 



in tlie case of human beings, but as will presently appear 

 there are some strains of flies which give curves almost 

 identical in this respect with the human mortality curves. 

 In the life curves of Figure 47, all different degrees of 

 inherited or constitutional variation in longevity are in- 

 cluded together. More accurate pictures of the true state 

 of affairs will appear when we come, as we presently 

 shall, to deal with groups of individuals more homoge- 

 neous in respect of their hereditary constituents. 



Having now demonstrated that the incidence of mor- 

 tality is in general similar in the fly Drosophila to what 

 it is in man, with a suitable change of unit of measure, 

 we mayproceed to examine some of the evidence regarding 

 the inheritance of duration of life in this organism. 

 The first step in such an examination is to determine 

 what degree of natural variation of an hereditary 

 sort exists in a general fly population in respect of this 

 characteristic. In order to do this it is necessary to 

 isolate individual pairs, male and female, breed them 

 together and see whether, between the groups of offspring 

 so obtained, there are genetic differences in respect of 

 duration of life which persist through an indefinite num- 

 ber of generations. This approaches closely to the pro- 

 cess called by geneticists the testing of pure lines. In 

 such a process the purpose is to reduce to a minimum 

 the genetic diversity which can possibly be exhibited in 

 the material. In a case like the present, the whole amount 

 of genetic variation in respect of duration of life which can 

 appear in the offspring of a single pair of parents is only 

 that which can arise by virtue of its prior existence in the 

 parents themselves individually, and from the combina- 

 tion of the germinal variation existing in the two parents 

 one with another. We may call the offspring, through 

 successive generations, of a single pair of parents a line 



