The history of the struggle against compulsion in vaccination is very interesting, as a strong condemnation of that arrogance of false science which presumes to trample on human rights whilst neglecting hygienic conditions. As all intelligent persons should be able to form a practical judgment on the important question at issue, I should like to dwell a moment on the subject of immunity, a fact (though now misapplied) on which compulsory vaccination is based. _Immunity._--Observation has long shown us that when the human system is gradually exposed to injurious influences, a certain tolerance of those influences may be acquired, which often enables those exposed to them to escape immediate death, although with impaired health, whilst healthy persons suddenly exposed to the same injurious influences die. This is a well-known fact, capable of abundant verification. Thus, persons long resident in a badly-drained house, although frequently ailing in various ways, may never be laid up with typhoid fever; a certain immunity has been obtained by the slow adaptation of the system to bad air, but at the sacrifice of vigorous health. But if a new and healthy family move into the same house a deadly outbreak of typhoid or diphtheria may at once result. In the malarious districts of the United States a large scattered population of what are called by the negroes ‘mean whites’ continue to live, with clay-coloured faces, enlarged spleens, and impaired vitality, yet for a stranger to sleep in those regions is deadly. The strong tendency to live, which we call vitality, though it has enabled those born and brought up under injurious influences to struggle on through life, does not prove equal to resistance in many constitutions suddenly exposed to the injurious influences. The medical statistics of our army in India show that the newly-arrived is far more apt to suffer from enteric fever than one who has been long in the country. ‘The percentage of deaths from this cause is nearly fivefold greater in the first or second year of service than from the sixth to the tenth year. Medical officers are unable to trace out in any given instance a definite insanitary condition to which with certainty the outbreak can be attributed.’ There is, therefore, fact for theory to be built on--viz., the possible adaptation of the human constitution to injurious influences, an adaptation which, whilst impairing general vigour, often produces immunity from rapid death.