66 Animal Life and Intelligence. 



meaning from these figures. I have, therefore, plotted in 

 the measurements for four other species of bats in tabular 

 form (Figs. 18, 14, 15, 16). 



Fig. 18, for example, deals with the common large 

 noctule bat, which may often be seen flying high up on 

 summer evenings. Now, the mean length of the radius and 

 ulna in eleven individuals was 51'5 millimetres. Suppose 

 all the eleven bats had this bone (for the two bones form 

 practically one piece) of exactly the same length. There 

 would then be no variation. We may express this supposed 

 uniformity by the straight horizontal line running across 

 the part of the figure dealing with the radius and ulna. 

 Practically the eleven bats measured did not have this 

 bone of the same length ; in some of them it was longer, 

 in others it was shorter than the mean. Let us run 

 through the eleven bats (which are represented by the 

 numbers at the head of the table) with regard to this bone. 

 The first fell below the average by a millimetre and a half, 

 the length being fifty millimetres. This is expressed in the 

 table by placing a dot or point three quarters of a division 

 below the mean line. Each division on the table represents 

 two millimetres, or, in other words, the distance between 

 any two horizontal lines stands for two millimetres 

 measured. Half a division, therefore, is equivalent to one 

 measured millimetre ; a quarter of a division to half a milli- 

 metre. The measurements are all made to the nearest half- 

 millimetre. The second bat fell short of the mean by one 

 millimetre. The bone measured 50*5 millimetres. The 

 third exceeded the mean by a millimetre and a half ; the 

 fourth, by three millimetres and a half. The fifth was a 

 millimetre and a half above the mean; and the sixth 

 and seventh were both half a millimetre over the mean. 

 The eighth fell short by half a millimetre ; the ninth and 

 tenth by a millimetre and a half ; and the eleventh by 

 two millimetres and a half. The points have been con- 

 nected together by lines, so as to give a curve of variation 

 for this bone. 



The other curves in these four tables are drawn in exactly 



