iv PREFACE 



that would like to have a quite modern and quite 

 simple account of this wonderful thought which now 

 runs through the whole of our culture. They want 

 the A B C of the matter, but they want it up-to-date, 

 so that they may understand what scientific men are 

 talking about to-day. I trust that this little work 

 will meet their wishes. 



J. M. 



