it。
The theory of the Organicists； like that of the Animists and Vitalists； agreed with the other two that vital activity could not be explained by the laws of physics and chemistry； but； unlike them； it held that it was a part of the structure of the body itself。 Naturally the practical physicians were more attracted by this tangible doctrine than by vague theories 〃which converted diseases into unknown derangements of some equally unknown 'principle。' 〃
It is perhaps straining a point to include this brief description of these three schools of medicine in the history of the progress of the science。 But； on the whole； they were negatively at least prominent factors in directing true progress along its proper channel； showing what courses were not to be pursued。 Some one has said that science usually stumbles into the right course only after stumbling into all the wrong ones； and if this be only partially true； the wrong ones still play a prominent if not a very creditable part。 Thus the medical systems of William Cullen （1710…1790）； and John Brown （1735…1788）； while doing little towards the actual advancement of scientific medicine； played so conspicuous a part in so wide a field that the 〃Brunonian system〃 at least must be given some little attention。
According to Brown's theory； life； diseases； and methods of cure are explained by the property of 〃excitability。〃 All exciting powers were supposed to be stimulating； the apparent debilitating effects of some being due to a deficiency in the amount of stimulus。 Thus 〃the whole phenomena of life； health； as well as disease； were supposed to consist of stimulus and nothing else。〃 This theory created a great stir in the medical world； and partisans and opponents sprang up everywhere。 In Italy it was enthusiastically supported； in England it was strongly opposed； while in Scotland riots took place between the opposing factions。 Just why this system should have created any stir； either for or against it； is not now apparent。
Like so many of the other 〃theorists〃 of his century； Brown's practical conclusions deduced from his theory （or perhaps in spite of it） were generally beneficial to medicine； and some of them extremely valuable in the treatment of diseases。 He first advocated the modern stimulant； or 〃feeding treatment〃 of fevers； and first recognized the usefulness of animal soups and beef…tea in certain diseases。
THE SYSTEM OF HAHNEMANN
Just at the close of the century there came into prominence the school of homoeopathy； which was destined to influence the practice of medicine very materially and to outlive all the other eighteenth…century schools。 It was founded by Christian Samuel Friedrich Hahnemann （1755…1843）； a most remarkable man； who； after propounding a theory in his younger days which was at least as reasonable as most of the existing theories； had the misfortune to outlive his usefulness and lay his doctrine open to ridicule by the unreasonable teachings of his dotage；
Hahnemann rejected all the teachings of morbid anatomy and pathology as useless in practice； and propounded his famous 〃similia similibus curantur〃that all diseases were to be cured by medicine which in health produced symptoms dynamically similar to the disease under treatment。 If a certain medicine produced a headache when given to a healthy person； then this medicine was indicated in case of headaches； etc。 At the present time such a theory seems crude enough； but in the latter part of the eighteenth century almost any theory was as good as the ones propounded by Animists； Vitalists； and other such schools。 It certainly had the very commendable feature of introducing simplicity in the use of drugs in place of the complicated prescriptions then in vogue。 Had Hahnemann stopped at this point he could not have been held up to the indefensible ridicule that was brought upon him； with considerable justice； by his later theories。 But he lived onto propound his extraordinary theory of 〃potentiality〃that medicines gained strength by being dilutedand his even more extraordinary theory that all chronic diseases are caused either by the itch； syphilis； or fig…wart disease； or are brought on by medicines。
At the time that his theory of potentialities was promulgated； the medical world had gone mad in its administration of huge doses of compound mixtures of drugs； and any reaction against this was surely an improvement。 In short； no medicine at all was much better than the heaping doses used in common practice； and hence one advantage； at least； of Hahnemann's methods。 Stated briefly； his theory was that if a tincture be reduced to one…fiftieth in strength； and this again reduced to one…fiftieth； and this process repeated up to thirty such dilutions； the potency of such a medicine will be increased by each dilution； Hahnemann himself preferring the weakest； or； as he would call it； the strongest dilution。 The absurdity of such a theory is apparent when it is understood that long before any drug has been raised to its thirtieth dilution it has been so reduced in quantity that it cannot be weighed； measured； or recognized as being present in the solution at all by any means known to chemists。 It is but just to modern followers of homoeopathy to say that while most of them advocate small dosage； they do not necessarily follow the teachings of Hahnemann in this respect； believing that the theory of the dose 〃has nothing more to do with the original law of cure than the psora （itch） theory has； and that it was one of the later creations of Hahnemann's mind。〃
Hahnemann's theory that all chronic diseases are derived from either itch； syphilis； or fig…wart disease is no longer advocated by his followers； because it is so easily disproved； particularly in the case of itch。 Hahnemann taught that fully three…quarters of all diseases were caused by 〃itch struck in；〃 and yet it had been demonstrated long before his day； and can be demonstrated any time； that itch is simply a local skin disease caused by a small parasite。
JENNER AND VACCINATION
All advances in science have a bearing； near or remote； on the welfare of our race； but it remains to credit to the closing decade of the eighteenth century a discovery which； in its power of direct and immediate benefit to humanity； surpasses any other discovery of this or any previous epoch。 Needless to say； I refer to Jenner's discovery of the method of preventing smallpox by inoculation with the virus of cow…pox。 It detracts nothing from the merit of this discovery to say that the preventive power of accidental inoculation had long been rumored among the peasantry of England。 Such vague； unavailing half…knowledge is often the forerunner of fruitful discovery。
To all intents and purposes Jenner's discovery was original and unique。 Nor； considered as a perfect method； was it in any sense an accident。 It was a triumph of experimental science。 The discoverer was no novice in scientific investigation； but a trained observer； who had served a long apprenticeship in scientific observation under no less a scientist than the celebrated John Hunter。 At the age of twenty…one Jenner had gone to London to pursue his medical studies； and soon after he proved himself so worthy a pupil that for two years he remained a member of Hunter's household as his favorite pupil。 His taste for science and natural history soon attracted the attention of Sir Joseph Banks； who intrusted him with the preparation of the zoological specimens brought back by Captain Cook's expedition in 1771。 He performed this task so well that he was offered the position of naturalist to the second expedition； but declined it； preferring to take up the practice of his profession in his native town of Berkeley。
His many accomplishments and genial personality soon made him a favorite both as a physician and in society。 He was a good singer； a fair violinist and flute…player； and a very successful writer of prose and verse。 But with all his professional and social duties he still kept up his scientific investigations； among other things making some careful observations on the hibernation of hedgehogs at the instigation of Hunter； the results of which were laid before the Royal Society。 He also made quite extensive investigations as to the geological formations and fossils found in his neighborhood。
Even during his student days with Hunter he had been much interested in the belief； current in the rural districts of Gloucestershire； of the antagonism between cow…pox and small…pox； a person having suffered from cow…pox being immuned to small…pox。 At various times Jenner had mentioned the subject to Hunter； and he was constantly making inquiries of his fellow…practitioners as to their observations and opinions on the subject。 Hunter was too fully engrossed in other pursuits to give the matter much serious attention； however； and Jenner's brothers of the profession gave scant credence to the rumors； although such rumors were common enough。
At this time the practice of inoculation for preventing small…pox； or rather averting the severer forms of the disease； was widely practised。 It was customary； when there was a mild case of the disease； to take some of the virus from the patient and inoculate persons who had never had the disease； producing a similar attack in them。 Unfortunately there were many objections to this practice。 The inoculated patient frequently developed a virulent form of the disease and died； or if he recovered； even after a mild attack； he was likely to be 〃pitted〃 and disfigured。 But； perhaps worst of all； a patient so inoculated became the source of infection to others； and it sometimes happened that disastrous epidemics were thus brought about。 The case was a most perplexing one； for the awful scourge of small…pox hung perpetually over the head of every person who had not already suffered and recovered from it。 The practice of inoculation was introduced into England by Lady Mary Wortley Montague （1690…1762）； who had seen it practised in the East； and who announced her intention of 〃introducing it into England in spite of the doctors。〃
From the fact that certain persons； usually milkmaids； who had suffered from cow…pox seemed to be immuned to small…pox； it would seem a