Let us take, for instance, the case of cancer of the lip. We see an old man who for years has been smoking a clay pipe. The stem of the pipe gets shorter as the years go by, and consequently, as he smokes it, hotter and hotter. One day he notices that his lip is cracked, the crack being just on that part with which he habitually holds his pipe. If we were to look at this under the microscope we should just see that the mucous membrane was broken at this point. Perhaps he stops smoking for a day or two till his lip has healed, and then continues to smoke again. Soon, from force of habit, the pipe returns to its old comfortable spot; and again the lip cracks. This time it is not so painful, and takes longer to heal. This cracked lip may be present for years, and if, after some time, we were to look at it again under the microscope, we should see a very different kind of thing. All round the crack would be congregated thousands of white blood cells, trying vainly to assist the sore to heal, but, as well as this, we should notice that, in their efforts to bridge the gap of broken mucous membrane, the delicate epithelial cells which line our lips had increased in number and thickness. We might also see that they had a tendency to grow down to the deeper layers of the lip. If we were to persuade our friend to give up his clay pipe and indulge in some other form of smoking, or even to have a few teeth extracted so that his pipe was more comfortable in some other position, the small ulcer would, given time and a little attention, heal up quite satisfactorily. But, with all the perversity of human nature, he will not; he only has a small sore: it doesn’t hurt him, or anyone else, so why should he worry?