138 THE THEORY OF MUTATION 
is constructed. The entries in this table show the 
fastest times recorded for the feat of trotting a measured 
mile in various years beginning with 1818. The ver- 
tical scale contains the times, which vary from three 
minutes down to one minute fifty-six seconds, and the 
horizontal scale shows the year in which the record was 
3,004 
Year 1820 “30 ‘40 © ©6"50 60 ‘70 "80 °90 1900 
FIG, 11. 
The figures to the left of the diagram are to be read as minutes and 
seconds. 
made. Some part of the improvement shown is clearly 
to be associated with better tracks, improved methods 
of training, etc., but these will scarcely affect the 
general character of the improvement due to selec- 
tion. As may readily be seen from the diagram, the 
improvement is at first rapid, but afterwards becomes 
