And fourthly, careful inspection of the siftings of the straw disclosed living parasites, small mites, which when applied to the skin quickly produced the characteristic eruption.

SANITATION

When the thoughtful historian gets far enough away from the nineteenth century to see it as a whole, no single feature will stand out with greater distinctness than the fulfilment of the prophecy of Descartes that we could be freed from an infinity of maladies both of body and mind if we had sufficient knowledge of their causes and of all the remedies with which nature has provided us.  Sanitation takes its place among the great modern revolutions—­political, social and intellectual.  Great Britain deserves the credit for the first practical recognition of the maxim salus populi suprema lex.  In the middle and latter part of the century a remarkable group of men, Southwood Smith, Chadwick, Budd, Murchison, Simon, Acland, Buchanan, J.W.  Russell and Benjamin Ward Richardson, put practical sanitation on a scientific basis.  Even before the full demonstration of the germ theory, they had grasped the conception that the battle had to be fought against a living contagion which found in poverty, filth and wretched homes the conditions for its existence.  One terrible disease was practically wiped out in twenty-five years of hard work.  It is difficult to realize that within the memory of men now living, typhus fever was one of the great scourges of our large cities, and broke out in terrible epidemics—­the most fatal of all to the medical profession.  In the severe epidemic in Ireland in the forties of the last century, one fifth of all the doctors in the island died of typhus.  A better idea of the new crusade, made possible by new knowledge, is to be had from a consideration of certain diseases against which the fight is in active progress.

Nothing illustrates more clearly the interdependence of the sciences than the reciprocal impulse given to new researches in pathology and entomology by the discovery of the part played by insects in the transmission of disease.  The flea, the louse, the bedbug, the house fly, the mosquito, the tick, have all within a few years taken their places as important transmitters of disease.  The fly population may be taken as the sanitary index of a place.  The discovery, too, that insects are porters of disease has led to a great extension of our knowledge of their life history.  Early in the nineties, when Dr. Thayer and I were busy with the study of malaria in Baltimore, we began experiments on the possible transmission of the parasites, and a tramp, who had been a medical student, offered himself as a subject.  Before we began, Dr. Thayer sought information as to the varieties of mosquitoes known in America, but sought in vain:  there had at that time been no systematic study.  The fundamental study which set us on the track was a demonstration