lation live in the towns of 100,000 inhabitants and upwards. In Scotland 30 per cent., and in England and Wales 54 per cent.; and we find the mortality from zymotic diseases to be roughly proportional to these figures. "We see here unmistakable cause and effect. Impure air, with all else that overcrowding implies on the one hand, higher death-rate on the other. This explains the constant difference between London and rural mortality, and it also explains what seems to have puzzled the Commissioners more than anything else—the intractability of some of the zymotics to ordinary sanitation, as in the case of measles especially, and in a less degree of whooping-cough—for in their case the continual growth of urban as opposed to rural populations has neutralised the effects of such improved conditions as we have been able to introduce.
But the most important fact for our present purpose is, that small-pox is subject to this law just as are the other zymotics, while it pays no attention whatever to vaccination. The statistician to the Registrar-General for Scotland gave evidence that ever since 1864 more than 96 per cent, of the children born have been vaccinated or had had previous small-pox, and he makes no suggestion of any deficiency that can be remedied. But in the case of Ireland the medical commissioner for the Local Government Board for Ireland, Dr. MacCabe, told the Commissioners that vaccination there was very imperfect, and that a large proportion of the population was " unprotected by vaccination," this state of things being due to various causes, which he explained (2nd Rep., QQ. 3,059-3,075). But neither Dr. MacCabe nor the Commissioners notice the suggestive, and from their point of view alarming, fact that imperfectly vaccinated Ireland had had far less small-pox mortality than thoroughly well-vaccinated Scotland, enormously less than well-vaccinated England, and overwhelmingly less than equally well-vaccinated London. Ireland—Scotland—England—London—a graduated series in density of popula-
