First of all, that one of the most powerful influences conditioning this isolation and revolt of the cells is age, both of the individual and of the organ concerned. Not only does far the heaviest cancer mortality fall between the ages of forty-five and sixty, but the organs most frequently and severely attacked are those which between these years are beginning to lose their function and waste away. First and most striking, the mammary gland and the uterus in women, and the shriveling lips and tongue of elderly men. To put it metaphorically, the mammary gland and the uterus, after the change of life, the lip, after the decay of the teeth, have done their work, outlived their usefulness, and are being placed upon a starvation pension by a grateful country. Nineteen out of twenty accept the situation without protest and sink slowly to a mere vegetative state of existence, but, in the twentieth, some little knot of cells rebel, revert to an ancestral power of breeding rapidly to escape extinction, begin to make ravages, and cancer is born. The age-preferences are well marked. Cancer is emphatically a disease of senility, of age; but, as Roger Williams has pointed out in his admirable monograph, not of "completed" senility. To express it in percentages, barely twenty per cent of the cases occur before forty years of age, sixty per cent between forty and sixty, and twenty per cent between sixty and eighty. Thus the early period of decline, the transition stage between full functional vigor and declared atrophy (wasting) of the glands, is clearly the period of greatest danger; precisely the period in which the gland-cells, though losing their function,--and income,--have still the strength to inaugurate a rebellion, and a sufficient supply of the sinews of war, either in their own possession or within easy striking distance in the tissues about them, to make it successful. Not less than sixty-five to seventy-five per cent of all cancers in women occur in atrophying organs, the uterus and mammary glands. A rather alluring suggestion was made by Cohnheim, years ago, that cancers might be due to the sudden resumption of growth on the part of islands or _rests_ of embryonic tissue, left scattered about in various parts of the body. But these are now believed to play but a small part, if indeed any, in the production of true cancer. Finally, what can be done to prevent or cure this grotesque yet deadly process? So far as it is conditioned by age, it is, of course, obvious that little can be done, for not even the most radical vivisector would propose preventing in any way as large a proportion as possible of the human race from reaching fifty or sixty, or even seventy years, to avoid the barely six per cent liability to cancer after forty-five.