But this inspiriting outlook which was perhaps the most comprehensive result of Darwin's work had indirect consequences that were vitally significant to education. It is with mental and not with physical development that education is primarily concerned, and yet mental development is now known to depend fundamentally upon physical forces. The same decade that witnessed the publication of the _Origin of Species_ also witnessed the birth of another great book, little known except to the specialist, and yet destined to achieve immortality. This book is the _Elements of Psychophysics_, the work of the German scientist Fechner. The intimate relation between mental life and physical and physiological forces was here first clearly demonstrated, and the way was open for a science of psychology which should cast aside the old and threadbare raiment of mystery and speculation and metaphysic, and stand forth naked and unashamed.
But all this was only preparatory to the epoch-making discoveries that have had so much to do with our present attitude toward education. The Darwinian hypothesis led to violent controversy, not only between the opponents and supporters of the theory, but also among the various camps of the evolutionists themselves. Among these controversies was that which concerned itself with the inheritance of acquired characteristics, and the outcome of that conflict has a direct significance to present educational theory. The principle, now almost conclusively established,[5] that the characteristics acquired by an organism during its lifetime are not transmitted by physical heredity to its offspring, must certainly stand as the basic principle of education; for everything that we identify as human as contrasted with that which is brutal must look to education for its preservation and support. It has been stated by competent authorities that, during the past ten thousand years, there has been no significant change in man's physical constitution. This simply means that Nature finished her work as far as man is concerned far beyond the remotest period that human history records; that, for all that we can say to-day, there must have existed in the very distant past human beings who were just as well adapted by nature to the lives that we are leading as we are to-day adapted; that what they lacked and what we possess is simply a mass of traditions, of habits, of ideals, and prejudices which have been slowly accumulated through the ages and which are passed on from generation to generation by imitation and instruction and training and discipline; and that the child of to-day, left to his own devices and operated upon in no way by the products of civilization, would develop into a savage undistinguishable in all significant qualities from other savages.