The most important of these is chronic irritation. We find that almost every cancer is preceded for a longer or shorter period by what may be called a precancerous condition. The more our knowledge increases the more we are finding out that this holds good. The commonest sites for cancer are the womb, the breast, and the stomach. These together account for more than sixty per cent. of all cancers, and far below them in frequency we find the tongue, the lip, and the bowel, and the various glands. Cancer of the womb is constantly preceded for many years by disease, palpable and curable, often the result of childbearing, and the part where it occurs is one bathed in an acid medium. Cancer of the breast also is constantly associated with preceding chronic inflammation, this condition itself producing, as one of its by-products, a highly acid substance, further to irritate the delicate cells already near the end of their tether. Mechanical irritation, beyond a doubt, is an important factor. Although in civilised countries the disease is distressingly common, in those countries where the breasts are habitually uncovered, cancer of this organ is extremely rare. The habitual friction of modern clothes predisposes cell-growth, infection from no matter what source is given a foothold, and after years of abuse, the cells lose the impulse to normal reaction and at last turn and slay their victim. There is evidence that about two-thirds of all the cases of cancer of the stomach originate in an old gastric ulcer, and the constant eating of hot food is perhaps enough to account for the remaining third. The delicate gastric cells, more abused than any other cells in the body, are bathed in a highly acid medium. It is no wonder that departure from their appointed path accounts for thirty per cent. of all cancers in men, and in women as well, if we except the two conditions just mentioned. In cancer of the kidney, the bladder, and the gall bladder, stones are nearly always present to initiate the irritation. In cancer of the tongue, syphilitic or other preceding conditions are nearly always there, whether it be the irritation from raw alcohol, hot tobacco smoke, or a broken tooth. It is interesting to note that, until syphilis appeared in Europe, cancer of the tongue was practically unrecorded in the existing literature. We have no need to go any further for examples of these precancerous irritative conditions. They are all curable or removable, but, as they do not as a rule give rise to acute painful symptoms, severely inconveniencing the patient, they are difficult to treat, and the unfortunate patient is told to _wait and see_, and is given medicine which may for a while relieve, but which――alas!――seldom has a chance to cure, or to prevent the fate which is slowly overtaking him.