vi Preface 
It has seemed best to leave the texts unaltered except 
for the correction of typographical errors, renumbering 
of tables and figures, and redrawing the latter. In a 
few places, where the original text has been found likely 
to be misunderstood, brief notes have been added. It is 
hard to resist the impulse to temper the style, especially 
of the ‘Animal Intelligence,’ with a certain sobriety and 
restraint. What one writes at the age of twenty-three 
is likely to irritate oneself a dozen years later, as it doubt- 
less irritated others at the time. The charitable reader 
may allay his irritation by the thought that a degree of 
exuberance, even of arrogance, is proper to youth. 
To the reports of experimental studies are added two 
new essays dealing with the general laws of human and 
animal learning. 
JANUARY, IQII. 
