Another very widespread delusion productive of great harm is that cancer is constantly associated with _wasting_, and makes _rapid progress_. These two symptoms are constantly associated with the disease in its latest stages but are not seen at all in early cases. One frequently hears people say that cancer is contagious, and also that it is hereditary. These two popular conceptions probably have the same basis. As we have seen, cancer is a very common disease, and it would be strange indeed if, putting all question of relationship on one side, we were not to see it quite commonly occurring in one or more members of the same family, and if occasionally we did not find a house in which each successive occupant for some years had cancer. I will leave it to the mathematicians to work out the probability of cancer occurring more than once in any given family. The necessary figures are easily obtained from the Registrar-General’s office. As far as I know, there is nothing truly in the nature of what may be called _evidence_ in support of either of these notions. Time after time people have described parasites of some kind as associated with cancer, but none of them has yet been made to answer to any of the tests necessary to establish anything more than a casual correlation. It may turn out to be that the causal agent in cancer formation is a parasite either visible under the microscope, or, what is more likely, belonging to the group of ultravisible, or filter-passing, organisms; but even if this be so, there are two other factors of immense importance, found so constantly associated with the disease, that their significance cannot be underestimated by anyone whose outlook is any wider than that of the mere purveyor of prescriptions. These two factors may be considered in a little more detail, as they are of importance with regard to the question of prevention. They are (1) the presence of an acid environment, and (2) what, for want of a better term, may be called chronic irritation. Whatever the prime cause may turn out to be, these can never be left out of account in any consideration of aetiology, and even if some specific cause is found, the discovery will not shake the validity of my thesis. For two thousand years people have speculated about the origin of cancer. Galen held a theory somewhat analogous to the present Chinese doctrine of the _yin_ and the _yang_; he taught, in essence, that some kind of “ch’i” had got at loggerheads with its fellow gases, and that the result was a general disturbance of bodily functions. Paracelsus thought that the salt balance of the body was upset, and textbooks still sometimes put this into modern medical terminology, saying that the balance of power between different types of cells is disturbed. This may or may not _describe_ what happens, but it is a long way from _explaining_ it.