An Introduction to a Biology 



together by a 6-foot chain. If one of them wishes to sleep 

 over the the other has to lie in the middle of the stall 

 over 6. If after a time the latter insists on moving to 12 

 the former must put up with 6. And similarly with inter- 

 mediate positions. This parallel illustrates only the maxi- 

 mum possible difference between first and second throws 

 in " connected " and " unconnected " pairs. To find a 

 parallel for the most usual difference between first and 

 second throws in connected pairs we should have to imagine 

 the leash connecting the dogs to be made of a piece of elastic 

 with a maximum stretch of 6 feet. 



We must return to the dice. Let us make a number of 

 pairs of such connected throws, and see what the result is. 

 On pages 224-225 are given the results of 500 such pairs. 

 The Roman numerals at the top of each column mean that 

 the left-hand figures give the results of the first throws ; the 

 right-hand ones those of second throws. 



The list does not show very much in this form. If you 

 look through it you will find that a high number is as a 

 rule followed by a fairly high one, and that a low one is 

 usually followed by a fairly low one. But this is not pre- 

 sented at all vividly to the eye. What we want is some 

 means of finding out, without the labour of counting through 

 the whole series, the number of times a given result in a 

 first throw is followed by a given result in a second. This 

 want is supplied by the so-called Correlation Table. Here 

 is one (see next page) on which are exhibited the results of 

 the 500 pairs of throws detailed on pages 224-225: 



The Table proper is bounded at the left and top by 

 single lines, and at the right and bottom by double ones. 



It is made up of 169 squares. A horizontal series of 

 these squares is spoken of as a row ; a vertical series of them 

 as a column. So that we may say that the table consists at 

 once of 13 horizontal rows, each of which is made up of 13 

 squares ; and of 1 3 vertical columns, each of which is like- 

 wise made up of 13 squares. 



