Now it is just this type of cancer that we have the opportunity to imitate in the laboratory. Dr Leitch, of the Cancer Hospital, has taken rats, guinea-pigs and rabbits; and, day after day for months, soot, tar, oils and all the irritants he could think of were respectively painted on some selected part of their bodies. At the Cancer Hospital he started using tar to paint on the under surface of the bodies of white mice. This was done every morning for several months, and, in a large percentage of cases, small warts were produced. The fate of these warts varied; some of them disappeared, but others progressed to the formation of true cancer. The results of these experiments made it extremely probable that the irritants were the direct cause of the cancer. Of course it is not _proved_, for it is possible to assume that there is some ubiquitous “other cause”, only waiting till the tissue resistance is lowered enough by the irritants to get its chance to act. Another interesting fact, which transpired as the result of this work, is that some of the animals from whom the warts disappeared developed cancer a month or so subsequent to the disappearance, thus showing that the _predisposition_ to cancer formation is acquired long before the growth actually appears. In human beings, the process of cancer formation in response to chemical irritants takes much longer (often twenty to thirty years), and is preceded by much the same sort of preliminary skin reaction as in animals. In looking for a proximal cause for cancer production, we should not, I think, look for a common cause in all cases, but should try to find something or _anything_ which will produce the necessary previous irritation. It has not, I think, been established beyond a doubt that chronic irritation is the _sole_ exciting cause of cancer――this in the nature of things would be very difficult to prove――but it has been shewn that its presence strongly predisposes to new growth formation. The problem which now arises is that of how we are going to put this knowledge we have gained to practical use in the prevention of cancer. In order to solve this we will consider in some detail the three commonest cancers met with, namely cancer of the breast, the womb and the stomach, and we will see how the problem applies to them. Now in cancer of the breast we have this outstanding fact that, almost all the cases show for some years beforehand obvious signs of chronic inflammation of the breast, and in nearly all of them this precancerous stage can be seen, when they are examined microscopically.