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But the belief that the disease is essentially one of the lymphatic system generally in connection with the theory of respiratory-infection would almost necessitate the idea that the lymphatic system was invaded directly from the lungs and not indirectly as part of a general septicæmic infection.

I find such a theory rather difficult of acceptance.

The greatest support for the theory of infection through the respiratory tract is sought for in the well established observations that plague has its greatest incidence in the ill-ventilated, dark, and dirty houses of the poorest quarters of infected places. In short it is in the overcrowded localities inhabited by the poorer classes that plague is worst. Overcrowding means ill-ventilation, it also in most cases ineans poverty. Poverty compels people to live in "slums.'.

Personal uncleanliness both as to dirt and vermin is the outcome of poverty in cities and Eastern villages. The necessity of living in cheap and therefore the most dilapidated and insanitary dwellings of cities is also a consequence of poverty.

In fact speaking generally overcrowding is inseperable from many other possi- ble factors in the spread of a disease such as plague.

This overcrowding has either directly or indirectly an undoubted influence on the spread of plague.

In Hongkong it has been noted during the last two epidemics that in the most overcrowded districts plague has been most prevalent.

Now overcrowding will increase the chances of the spread of plague whether the chief channel of infection be the skin, alimentary or respiratory system.

Whether the infection be conveyed by inoculation through the skin, by the contamination of food or food utensils, or by the breathing of air containing in- fective particles, it is obvious that a plague patient in an overcrowded room is a danger to more persons than he would be in a well ventilated apartment where intimate contact with other persons is less likely to happen.

It has been stated in support of the theory that plague spreads through the agency of "devitalised air".

devitalised air", that it is difficult to believe that abrasions of the skin sufficient to allow entry of the bacillus can be so common, or that infective material can be so widely distributed as to infect so many persons through the skin.*

The percentage of plague cases in Hongkong for the population of the two most overcrowded districts, namely, Health Districts 2 and 9 was in 1903 only 0′94 per cent. for Health District 2, and 0.73 per cent. for Health District 9; while in 1904 (a mild year) it was only 0.55 in No. 9 Health District (for the seven months dealt with in this report) which was by far the worst affected district in the City of Victoria.

Το any one who has seen the conditions under which the people live who inhabit the very poor and overcrowded districts in Hongkong it is not a matter of surprise that infective material can be widely enough distributed within a house in which there is found a case of plague to cause a case incidence of such a small percentage; nor is it difficult to believe that the small lesions of the skin necessary for the infection can be very common.

This evidence can scarcely be accepted against the skin-infection theory and per contra for the respiratory-tract theory,

The theory of respiratory-infection being founded on the supposed entry of the bacilli into the lungs with the inspired air, it becomes necessary to discuss the question of the existence of the bacilli in the form of dust in the atmosphere.

I can find no trustworthy record of the bacillus having been isolated from atmospheric dust. The bacillus does not withstand complete drying. Were it present in living form in the dust of houses, one might reasonably expect those employed in disinfecting to shew a marked plague incidence, which experience does not bear out.

* A Treatise on Plague by Major THOMSON, I.M.S., and Dr. JOHN THOMSON, 1901, page 115.