PREFACE AND INTRODUCTION vii 
the hypothetical nature of the thesis, they 
did not appear to be justified. Repetition 
has been avoided as much as possible, even 
at the expense of forward reference, so that 
the whole volume must be read in order that 
a clear understanding may be obtained. The 
volume is divided in the following manner: 
Chapter I states the Theory; Chapters II, 
IIT, and IV deal at length with some facts 
on which the Theory rests; the remaining 
chapters exemplify the Theory. 
Exception will probably be taken to some 
of the premises chosen, to some of the steps 
in the arguments; but entrance into this 
most difficult subject is made only with great 
diffidence. Nevertheless hope is entertained 
that even if the Theory does not correlate a 
good number of the facts to which it is applied, 
it will at least explain a few. Even if it does 
no more than lead other minds along new 
lines of thought, the author will not be dis- 
satisfied. 
