This book is the fruit of long experience and reflection. It has two fundamental ideas—the study of nature, and the study of the psychology of man in health and in disease.

To harmonize the aspirations of human nature and the data of the sociology of the different human races and the different epochs of history, with the results of natural science and the laws of mental and sexual evolution which these have revealed to us, is a task which has become more and more necessary at the present day. It is our duty to our descendants to contribute as far as is in our power to its accomplishment. In recognition of the immense progress of education which we owe to the sweat, the blood, and often to the martyrdom of our predecessors, it behoves us to prepare for our children a life more happy than ours.

I am well aware of the disproportion which exists between the magnitude of my task and the imperfections of my work. I have not been able to study as much as should be done the innumerable works which treat of the same subject. Others, better versed than myself in the literature of the subject, will be able later on to fill this regrettable lacuna. I have endeavored, above all things, to study the question from all points of view, in order to avoid the errors which result from any study which is made from one point of view only. This is a thing which has generally been neglected.

I must express my thanks to my friend, Professor Mahaim, and especially to my publisher and cousin, S. Steinheil, for the help and excellent advice which they have given me in the revision of my work; also to Professor Boveri, who has been kind enough to revise the figures, 1 to 17.

Dr. A. Forel.

Chigny près Morges (Suisse).