We have now established the bases for a practicable eugenics program. Men differ; these differences are inherited; therefore the make-up of the race can be changed by any method which will alter the relative proportions of the contributions which different classes of men make to the following generation. For applied eugenics, it is sufficient to know that mental and physical differences are inherited; the exact manner of inheritance it would be important to know, but even without a knowledge of the details of the mechanism of heredity, a program of eugenics is yet wholly feasible. It is no part of the plan of this book to enter into the details of the mechanism of heredity, a complicated subject for which the reader can refer to one of the treatises mentioned in the bibliography at the close of this volume. It may be worth while, however, to outline in a very summary way the present status of the question. As to the details of inheritance, research has progressed in the last few years far beyond the crude conceptions of a decade ago, when a primitive form of Mendelism was made to explain everything that occurred.[43] One can hardly repress a smile at the simplicity of those early ideas,--though it must be said that some students of eugenics have not yet outgrown them. In those days it was thought that every visible character in man (or in any other organism) was represented by some "determiner" in the germ-plasm; that by suitable matings a breeder could rid a stream of germ-plasm of almost any determiner he wished; and that the corresponding unit character would thereupon disappear from the visible make-up of the individual. Was a family reported as showing a taint, for instance, hereditary insanity? Then it was asserted that by the proper series of matings, it was possible to squeeze out of the germ-plasm the particular concrete _something_ of which insanity was the visible expression, and have left a family stock that was perfectly sound and sane.