Introduction.
This little work has been written in a hurry amid the worry and pressure of professional duties. It has also been ruthlessly cut down to compress it within the space allotted by the publisher. Neither profundity of matter, nor perfection of style is aimed at. It is written in every-day language, and may limp a little here and there. For none of these faults do I offer an apology to my readers. I ask them to take it as it is—as something more than a mere introduction to a most interesting and important subject.