THE LAWS OF HEREDITY

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.

The minute, meticulous researches of experimental breeders[44] have left such a view of heredity far behind. Certainly the last word has not been said; yet the present hypotheses work, whenever the conditions are such as to give a fair chance. The results of these studies have led to what is called the factorial hypothesis of heredity,[45] according to which all the visible characters of the adult are produced by (purely hypothetical) factors in the germ-plasm; it is the factors that are inherited, and they, under proper conditions for development, produce the characters. The great difference between this and the earlier view is that instead of allotting one factor to each character, students now believe that each individual character of the organism is produced by the action of an indefinitely large number of factors,[46] and they have been further forced to adopt the belief that each individual factor affects an indefinitely large number of characters, owing to the physiological interrelations and correlations of every part of the body.

HOW DO YOU CLASP YOUR HANDS?

Fig. 16.—If the hands be clasped naturally with fingers alternating, as shown in the above illustration, most people will put the same thumb—either that of the right or that of the left hand—uppermost every time. Frank E. Lutz showed (American Naturalist, xliii) that the position assumed depends largely on heredity. When both parents put the right thumb uppermost, about three-fourths of the children were found to do the same. When both parents put the left thumb uppermost, about three-fifths of the children did the same. No definite ratios could be found from the various kinds of matings. Apparently the manner of clasping hands has no connection with one's right-handedness or left-handedness. It can hardly be due to imitation for the trait is such a slight one that most people have not noticed it before their attention is called to it by the geneticist. Furthermore, babies are found almost always to clasp the hands in the same way every time. The trait is a good illustration of the almost incredible minuteness with which heredity enters into a man's make-up. Photograph by John Howard Paine.