The longevity of that remote period must have surpassed the age of the present time. From the circumstance that the greatest number of deaths occurs at the age of seventy years, Ebstein[316] has inferred that seventy years is the normal duration of life. Although there is no doubt but that the duration of human life has become longer in the nineteenth century, we must believe that it was still longer in Biblical times, a fact that is not particularly surprising. I have called attention to the important influence of syphilis in inducing premature and pathological old age, as that disease is a chief cause of arterial sclerosis and degeneration of the higher elements of the body. Syphilis has an influence still more serious because its effects are inherited. Now although the Bible refers to diseases of the genital organs and lays stress on circumcision, there is no direct evidence in it as to the existence of syphilis. Ebstein, in a treatise on the medicine of the Bible,[317] is confident that there is no reference to syphilis in that Book. Moreover, in the ancient world generally, syphilis was either unknown or existed only in an attenuated form. Haeser,[318] the author of the best modern treatise on the history of medicine, thinks that if syphilis did exist in the ancient world, it occurred in a localised form and did not become a general disease of the system as is the case among the moderns. Humanity would make a great stride towards longevity could it put an end to syphilis, which is the cause of one fifth of the cases of arterial sclerosis. The suppression of alcoholism, the second great factor in the production of senile degeneration of the arteries, will produce a still more marked extension of the term of life. Scientific study of old age and of the means of modifying its pathological character will make life longer and happier. Although modern knowledge is still imperfect, there is no reason to be pessimistic on the subject of old age. CHAPTER XI INTRODUCTION TO THE SCIENTIFIC STUDY OF DEATH Theory of the immortality of lower organisms—Immortality of the sexual cells in higher organisms—Immortality of the cellular soul—Occurrence of natural death in the case of certain animals—Natural death in the Ephemeridæ—Loss of the instinct of preservation in adult ephemerids—Instinct of life in the aged—Instinct of natural death in man—Death of old men in Biblical times—Changes in the instincts of man and lower animals