7>7^ 



MODERN THE0KIP:S OF HEREDITY. 



It is obviously also possible to speak of the " relative 

 frequency" as one (i) for the 5 heads, 5 tails, and as decimal.-. 

 {e.g., .84, .48, .18, .04, and .004) for the other possible com- 

 binations. The same relations may be calculated for larger 

 numbers, e.g., for 999 coins we have : — 



Heads. Tails. Relative Frequency. 



500 499 1 .000 



501 

 505 

 510 

 520 



540 

 560 



498 



494 

 489 



479 

 459 

 439 



996 

 942 

 803 

 432 



•037 

 .0006 etc. ; 



These results may also be expressed on a curve ; the central part 

 of such a curve is shown in Figure 3. 



Fig- 3- 



Such a curve as this is known as a ' normal " curve. The 

 longest perpendicular, M. {i.e., that at the 500:499 mark) is 

 called the " mode." If we erect a perpendicular, Q, in such a 

 position as to divide the area to the left of M, and enclosed by 

 the curve and its ordinates. into equal halves, then the horizontal 

 distance from Al to Q is called the " quartile:" 



Now a curve exactly similar to this may be constructed to 

 show the way in which certain characters are distributed in a 

 number of individuals. Take, for example, height in man. If 

 we take a tliousand men at random, and measure their heights 



