THE BIOLOGY OF DEATH 457 



Now it is altogether probable that one could get just as good a fit to 

 the observed d x line as is obtained by Pearson's five components by 

 using a 17 constant equation of the type 



y=a~\-bx-{-cx 2 -\-dx :i -\-ex' i -\-fx 5 -}-gx 6 -\- -\-nx 16 



and in that event one would be quite as fully justified lor really un- 

 justified) in concluding that the d x line was a homogeneous curve as 

 Pearson is in concluding from his five-component fit that it is com- 

 pound. 



Indeed Wittstein's formula involving but four constants gives sub- 

 stantially good fit over the whole range of life. 



But in neither case is the curve-fitting evidence, by and of itself, in 

 any sense a demonstration of the biological homogeneity or hetero- 

 geneity of the material. Of far greater importance, and indeed conclu- 

 sive significance, is the fact, to be brought out in a later paper in this 

 series, that in material experimentally known to be biologically homo- 

 geneous, a population made up of full brothers and sisters out of a 

 brother x sister mating and kept throughout life in a uniform environ- 

 ment identical for all individuals,o/ie gets a d x line in all its essential 

 features, save for the absence of excessive infant mortality arising from 

 perfectly clear biological causes, identical with the human d x line. 

 It has long been apparent to the thoughtful biologist that there was not 

 the slightest biological reason to suppose that the peculiar sinuosity of 

 the human d x line owed its origin to any fundamental heterogeneity 

 in the material, or differentiation in respect of the forces of mortality. 

 Now we have experimental proof, to be discussed in a later paper in 

 this series, that with complete homogeneity of the material, both genetic 

 and environmental, one gets just the same kind of d x line as in normal 

 human material. We must then, I think, come to the conclusion that 

 brilliant and picturesque as is Pearson's conception of the five Deaths, 

 actually there is no slightest reason to suppose that it represents any 

 biological reality, save in the one respect that his curve fitting demon- 

 strates, as any other equally successful would, that deaths do not occur 

 chaotically in respect of age, but instead in a regular manner capable 

 of representation by a mathematical function of age. 



