Explanations of the causation of cancer have been sought in many directions; and three chief theories have been set out. The most important, and the most interesting from the point of view of the practising physician, is that which considers cancer as provoked by long continued irritation under certain circumstances. This doctrine seems more “true” in respect of the Carcinomata――the cancers of the adult and the old, and of tissues in contact with the extra-personal world――than it is in respect of the Sarcomata――the cancers of the young, and of those inner parts not exposed to irritation by contact with the world. Yet sarcomata in real life do often seem to follow _injury_, and the tissues in which they form _may_ be obnoxious to injurious influences of which we know nothing. Another view is that cancer may be due to a parasite of some kind or another. Certainly, so far as some lower animals are concerned, this is true, for certain rat and mice cancers are now known definitely to be associated with parasites. But then we may say, and properly, that in such cases the parasites are merely acting as do other irritants, and are not “specific” causes of cancer. The third doctrine, or set of doctrines, regards cancers as arising when parts of the body (or rather, elements in the tissues of certain parts) no longer act in due subordination to the needs of the whole organism, but comport themselves “anti-socially”: developing irregularly; propagating themselves illegitimately; and so becoming parasitic to the commonwealth of the body. Those who hold this will admit that, in many cases, this revolutionary tendency is one provoked by irritation and the like: that sometimes it is a mere manifestation of irregular decay; and that, when it occurs in young subjects, it is because some islets of tissue have become misplaced, tucked away, ill-formed, and hampered in development, and so liable to provoke trouble later under stress of greater or less urgency. Such a view has much plausibility; there are flaws in a steel girder; there are tucked-in edges in even the best bound book, and there are developmental errors in most of us. Moreover, there is Dr Creighton’s doctrine of physiological resistance. A part not put to its proper use is more apt than another to become cancerous. Certainly, unmarried women are more liable than are married to suffer cancer of the breast or ovary. Yet married women are more apt than unmarried to suffer cancer of the womb. Are we to say that in these latter there has been physiological misuse, or irritation produced by unhealthy child-bearing? So far is the problem removed from simplicity!