1. We collected 27 pairs of twins (MZ 21, DZ 6) ranging from 41 to 80 years, and 16 pairs of homosexual siblings (SL) ranging from 62 to 90 years, all of whom were Japanese in the western district of Honshū-Island. First, we measured the bodily constitution, certain body parts and certain bones, and investigated their life- histories. Next, we investigated various senescent phenomena. That is: — the approximate number of gray hairs on the head, the appearance and the size of alopecia senilis in the parietal region, the approximate number of wrinkles in the frontal region, the comparative number of sulci cutis on the nape of neck, the back of the right hand, and the anterior chest, the development of senile pigment spots on the face, the back of the right hand, the extensor surface of the right forearm, and the upper half of the back, the appearance of the senile wart, the development of pinguecula, the development of arcus senilis, the development of marginal protuberances on the lumbar vertebral bodies, and the upper end of the tibia, the existence of fabella in the right knee, and the calcification in the wall of the arch of the aorta.

With regard to these items we calculated the middle percentage deviation, the coefficient of correlation, or the rate of concordance, and on the photographs or X-ray photographs the resemblance between both partners was examined.

2. We have concluded that the following items are mainly subject to the control of heredity. That is, the approximate number of gray hairs on the head, the existence, the localization and the shape of alopecia senilis on the parietal region in ♂, the localization and the shape of wrinkles on the frontal region, the localization and the development of senile pigment spots on the face, the development of pinguecula and of arcus senilis of the eye, the development and the shape of marginal protuberances on the lumbar vertebral body, and the development of marginal protuberances on condylus lateralis et medialis of the tibia. On the other hand, the following items were found to be fairly subject to influence of the environmental factors. That is, the appearance of alopecia senilis on the parietal region in ♀, the number of wrinkles on the frontal region, the comparative number of sulci cutis on the nape of neck, the back of the right hand, and the anterior chest, the number of senile pigment spots on the back of the right hand, the extensor surface of the right forearm, and the upper half of the back, the appearance of senile wart on the body, and the calcification in the arch of the aorta.

3. Concerning those senescent phenomena, which are strikingly subject to the control of heredity, a certain degree of difference between both partners of MZ was sometimes recognized. When the differences in the progression of various senescent phenomena between both partners of MZ were observed, it was often recognized that some senescent phenomena make more progress in one partner, while on the other hand the other ones do in another partner.

4. In order to decide certain environmental factors, which have influence upon the difference between both partners of MZ, we referred to the existence of heavy labor for long duration, the number of deliveries, the grade of the sunburn, the habit of alcoholic drinking, smoking, and history of various diseases. However, we could not decide that some of these factors had certain definite relationship with the progression of certain senescent phenomena. However, we might probably presume that the old-styled Japanese hair dressing worn for many years had a certain relationship with the appearance of the baldness in ♀.