It is these two layers which, as Mr Wright so aptly remarks, are in direct contact with the outer world. Now, while the carcinomata (which constitute the class of cancers chiefly discussed in this book) in general affect people who have passed the midpoint of life――those for whom, as Rabelais says, it is _midi passé_――the sarcomata, which are less common than the carcinomata, are rather more frequently, yet not exclusively, found in young people; in those indeed, who have not reached life’s apogee. It is important that these facts should be borne in mind, for generalisations founded upon the study of carcinomata alone cannot be necessarily true in respect of all Cancer, unless the use of the term cancer _be restricted_ to the class technically known as carcinoma. To say that Cancer can be prevented if constipation is avoided is clearly misleading, when we remember that quite young children, nay, infants, may be the subject of sarcoma; unless of course we define cancer, as some would do, as the kind of growth that, _ex hypothesi_, is prevented when constipation is avoided. It is confusion of this sort, bred by slovenly expression out of loose thinking, that is in great part responsible for the present bewilderment of the public. Another fertile source of confusion is the obscurity that attends both the popular and the professional use of the words “cause”, “causation”, and the like. The public demands that “the” cause of cancer be discovered, and is prepared to pay generously that this discovery be made. Unfortunately neither the public, nor men of science, care overmuch to discuss what they mean by cause and causation. This is no place in which to trench upon a province unsuccessfully explored by Locke, by Hume, and by Kant. Yet it is of vital importance that all doctors, scientists, and laymen should recognise two different _uses_ of these words.