4(3!) 



The Intensity of Natural Selection in Man. 

 By Karl Pearson, F.K.S. 



(Eeceived June 4, — Eead June 27, 1912.) 



(1) In a paper* communicated to the Eoyal Society in 1899, and later in 

 greater elaboration published in ' Biometrika/f 1901, it has been shown on 

 the basis of the inheritance of longevity that the selective death-rate in man 

 amounted to at least 60 per cent, to 80 per cent, of the total death-rate. The 

 matter has been recently reconsidered by Prof. Ploetz ; J who, dealing with 

 material wholly different from that of Beeton and Pearson came to similar 

 conclusions. The point is a very vital one, for, combined with : (i) the 

 heredity of physical and mental characters in man,| and (ii) the demonstra- 

 tion that the longer-lived have more offspring,|| we reach a definite knowledge 

 that Darwinism does apply, and very intensely applies, even to man under 

 civilised conditions. 



The difficulty of a direct investigation of the problem lies in securing 

 uniformity of environment. We have to demonstrate that when under the 

 same environment there is a heavier death-rate among a given group of human 

 beings, then among the survivors of this group in a given later period the 

 death-rate will be lessened. Now each group of individuals we attempt to 

 deal with has its own environment, and if that is a bad environment we 

 should expect to find a heavy death-rate both at the earlier and later 

 periods ; this obviously must obscure the action of natural selection. For 

 example in districts with a high infant mortality we might expect a high child 

 mortality, say deaths from two to five years of life, because a bad environment 

 sends up the intensity of both. The correlation between deaths in the first 

 year of life (0 — 1) and in the next four years of life (1 — 5) for a given district 

 will certainly be positive if no correction be made for varying environment. 

 Quite recently this matter has been discussed by determining the correlation 

 between the ages — 1 and 1 — 5 in the administrative counties of England 

 and Wales.1T As (a) the group — 1 was not followed to 1 — 5, but the 



* ' Roy. Soc. Proc.,' vol. 65, p. 290, et seq. 



t Vol. 1, p. 50, et seq.— especially pp. 74-5. 



1 ' Archiv fur Rassen- u. Gesellschafts-Biologie,' 1909, vol. 6, p. 33. 



§ " On the Laws of Inheritance in Man. I. — Inheritance of Physical Characters," 

 ' Biometrika,' vol. 2, p. 357 ; II. — " On the Inheritance of the Mental and Moral 

 Characters in Man," ' Biometrika,' vol. 3, p. 131. 



|| " On the Correlation between Duration of Life and Number of Offspring," ' Roy. 

 Soc. Proc.,' vol. 67, p. 159. 

 IT 4 Local Government Board Report,' Cd. 5263. 



