EVOLUTION 



409 



we see that selection has changed the type, the variabihty, 

 and even the sign of the skewness, and from a knowledge 

 of these changes the mathematician can determine a few 

 numerical constants which effectively describe the relative 

 death-rate of individuals having different values of the 

 character under consideration. Let us look at this again 

 from the graphical standpoint. The diagram gives by 

 the polygon i, d, a, h, i 5 the distribution of the character 

 at the first epoch, and by the polygon 2, c, b, g, 1 4 its 



Z -6 + 567 89 \^10 II 12 13 J4- J5 J6 /7 

 Scale of Character . 



Fig. 29. 



distribution at the second epoch, for our groups are 

 supposed to be fair samples of the total population. 

 Now since each of these distributions contains looo 

 members, the polygon 2, e, b, g, 14 cannot lie entirely 

 inside i, d, a, c, 15. The total death-rate reduces the 



original lOOO adults to say th of 1000, and we have 



accordingly to take -th of the vertical ordinates of 2 

 e, b, g, 14 in order to get the actual distribution of the 

 original lOOO adults after the interval. Consider then 

 the frequency of individuals with a character of scalar 



value 5 ; this is measured by ^/5 before and by -^5 



