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xviii the opportunities for man to contract the common disease are most frequent, or are only found, in the tropics. Such, most probably, are some of the tropical ringworms.

Certain parasites are so organized that before re-entering man they must pass a part of their lives as free organisms in the outer world, where they require a relatively high temperature for their development. Such parasites, therefore, and the diseases they give rise to, must necessarily be tropical or sub-tropical. The Ankylostomum duodenale and ankylostomiasis are an instance in point.

There is a class of intoxication diseases which depend on toxins generated by germs whose habitat is the soil, water, or other external media, and whose germs do not enter the human body as a necessary feature in their life-histories, although their toxins may. The yeast plant 'and its toxin, alcohol, and the disease it causes, alcoholism, are the most familiar example of this. Such, too, are ergotism, atriplicism, and perhaps lathyrism. These germs require certain temperatures and certain media; consequently the diseases they produce have a corresponding geographical range. If one of these conditions be a high temperature, the disease is mainly a tropical one.

Lastly, I can conceive, and believe, that there is another and less directly-acting set of conditions influencing the distribution of disease conditions which as yet have been ignored by epidemiologists, but which, it seems to me, must have an important bearing on this subject. Disease germs, their transmitting agencies, or their intermediate hosts, being living organisms, are, during their extracorporeal phases, necessarily competing organisms, and therefore liable to be preyed upon or otherwise crushed out by other organisms in the struggle for existence. The malaria parasite is absent in many places in which, apparently, all the conditions favourable to its existence are to be found in perfection. Why is it not found there, seeing that it must certainly have been frequently introduced? I would suggest that in some instances this and other disease germs, or the organ-