family in which a large proportion live to old age with unimpaired faculties. There you know is a good constitution in an inheritable form. On the other hand, you will find a family in which a large proportion die at birth and in which there are relatively few people who live to extreme old age. There has developed an hereditary weakness of constitution. Longevity is a guide to constitution." Not only does it show that one's vital organs are in good running order, but it is probably the only means now available of indicating strains which are resistant to zymotic disease. Early death is not necessarily an evidence of physical weakness; but long life is a pretty good proof of constitutional strength.
Dr. Bell has elsewhere called attention to the fact that, longevity being a characteristic which is universally considered creditable in a family, there is no tendency on the part of families to conceal its existence, as there is in the case of unfavorable characters—cancer, tuberculosis, insanity, and the like. This gives it a great advantage as a criterion for sexual selection, since there will be little difficulty in finding whether or not the ancestors of a young man or woman were long-lived.9
Karl Pearson and his associates have shown that there is a tendency to assortative mating for longevity: that people from long-lived stocks actually do marry people from similar stocks, more frequently than would be the case if the matings were at random. An increase of this tendency would be eugenically desirable.10 So much for the physique.
Though eugenics is popularly supposed to be concerned almost wholly with the physical, properly it gives most attention to mental traits, recognizing that these are the ones which most frequently make races stand or fall, and that attention to the physique is worth while mainly to furnish a sound body in which the sound mind may function. Now men and women may excel mentally in very many different ways, and eugenics, which seeks not to produce a uniform good type, but excellence in all desirable types, is not concerned to pick out any