Public health problems

Front matter

Tni Contemporary

Science Series

HAVELOCK ELLIS

Contents

INTRODUCTION I

Life and health — Personal and public health— Statistics and experiment — Education — Private habits and public measures — Ultimate objects — Public opinion — Modern pathology — Rapid advances — Questions of the hour — Application of scientific discoveries— Organisation at hand — Public health humanitarian — Problems involved — Their scope.

Chapter I

Evolution — Variation — Heterogeneity of progenitors — Impress of environment — Anabolism and kataholism — Predisposition of reproductive elements — Acquired characters — Recapitulation — Recrudescence — New combinations — Use and disuse — Augmentation of potential characteristics — Complexity of multicellular organisms — Influence of survival — Duration of external influences — Extreme effects of use and disuse — Acquired disease — Diathesis — Relative values of selection and adaptation — Survival — Natural selection — Sexual selection — Marriage — Adaptation — Habits — Education — The laws of health.

Chapter II

Light — Sunlight and daylight — Effect upon plants — Cryptogams — Action upon micro-organisms — Influence upon animals — Necessary to development — Effects upon man — Northern and southern races — Town and country dwellers— Effects upon the sick — Concomitants of deficient light — Artificial light. — Heat — Inequalities of transmission — Land and water — Latitude and altitude — Inland and insular climates — Mountains, forests, and soils— Isothermal lines — Flora and fauna — Range and extremes of temperature — Effects upon plants, microbes, animals, and man— Season — Mortality — Morbidity — Acclimatisation — Artificial temperatures.

VI CONTENTS.

PAGE

Chapter III

Air — Constituents — Gaseous and suspended impurities — Respiration— Cubic space — Overcrowding — Combustion — Smoke — Fog and mortality — Effects of artificial lighting and warming — Decomposition — Air of sewers, marshes, and factories. — Water — Moving and still — Effects upon climate — Potable and domestic water — Sources— Quantity — Effects of insufficiency and of impurities. — Soil — Contour — Structure — Pervious and impervious — Organic pollution — Nitrification — Microbes — Surface-water — Ground-water — Ground-air — Effects upon health.

BIOLOGICAL AGENTS 73

Physico-chemical and bio-chemical pathology — Inter-dependence of organic life in air, water, and soil — Plants — Microbes — Saprophytic and pathogenic —The destructive struggle — Parasites— Animals — Man— Coincident observation of diseases amongst plants, animals, and man.

Chapter I

111 -health and disease — Early ideas of disease — Leprosies— Confused diagnoses — Plagues — Chronological differentiation of communicable diseases— Early ideas of commuuicability — Biblical records — Uncleanness and contagiousness — Divine wrath and epidemicity — Supernatural, astronomical, and cosmic influences — Pestilences of the fourteenth century — Origin of the modern idea of communicability — Epidemic influences — Meteorological, telluric, and dietic causes — A materies morbi — Improvements in the microscope — Advances in pathology — Observations of bodily temperature — The cellular theory — Researches in inflammation — Microphytes in pus — The germ theory — A contagium vivum — Parallelism of phenomena of diseases and life histories of parasites — Recent views of disease causation.

Chapter II

Contagion and infection — Putrefaction and fermetation— Bacteriological discoveries — Parasitic diseases — Animal and vegetable parasites — The conditions of parasitism — Zoo-parasites

CONTENTS. VII

PAGH

— Ecto-, ento-, and hoemato-zoa — Reproduction of zoo-parasites— Diathesis verminosa and abiogenesis — Helminthiasis — Eilariensis — Malarial plasniodia — Phyto-parasites — El to-, ento-, and hnemato-phyta — Microphytio diseases — Reproduction of microphytes — Spore formation and vitality.

Chapter III

DISSEMINATION . . . . . . . . Il6

Media of survival of the contagia — Free and parasitic existence — Survival in dead and living organic matter — In air, water, and food — In plants, animals, and men — Anthropogenous, zoogenous, phytogenous, and saprogenous diseases — Modes of transport — By living organisms — By fomites, movable and fixed — Intermediaries— Direct and indirect infection — Contact — Inoculation — Inhalation — Ingestion — Air-borne and waterborne contagia — Distant transport — Modes of ingress — Seat of invasion — Access to the selective point — Susceptibility of the individual — Penetration of surface — By lesion existing or induced— Without lesion — Quantity of virus — Traversation of placenta — Effects within the body — Stages of disease- Incubation — Variation in artificial inoculation and natural infection — Invasion — Commencement and duration of infectiveness — Mycosis and toxaemia — Systems affected — Modes of egress and elimination — Exhalations, secretions, excretions, eruptions, and discharges — Variations according to seat of disease — Local distribution — Simultaneous, consecutive, and centrifugal outbreaks of disease — Effects of brief and prolonged precursive stage — Centrifugal extension and centripetal investigation — The proofs of centrifugal infection- Conditions of the rate of dissemination — Geographical distribution— Endemic and exotic diseases— sporadic, epidemic, and pandemic prevalence— Endo- and exo-epidemics — Diseases of temperate and tropical zones — Yellow fever — Asiatic cholera —Oriental plague— Leprosy.

Chapter IV

General and comparative mortality — Case fatality — Effects of season — Mortality maxima and minima — Prevalence and virulence of disease — Influence of social and hygienic conditions— Influence of sex, and of age — Variations of mortality in epidemics — Variations in individuals — Variations in contagia— Modifications of microbes - Attenuation and intensification— Pasteur's experiments — Effects of air, light, and chemical substances — Passage through living orgauisms — Regeneration — Origin of pathogenicity and communicability — Immunity — Congenital and acquired— Natural and artificial — Vital, exhaustion, antidote, and phagocyte theories — Products of bacteria — Chimiotaxis and phagocytosis— Biotic and toxic microbes — Toxines — Albumoses — Serum.

vill CONTENT?.

Part III. Defensive Measures Against Communicable Diseases

CHAPTER I. page

Forty-day period of ardent diseases — Initiation of quarantine — Severity of measures — Cordons— Extension of system — Modifications of methods — Indications of fallacies — Measures proposed as substitutes — Observation of the manner of the spread of diseases — Objections raised to quarantine — Treatment in detail versus exclusion in bulk— Sea-passages and overland routes — The gradual elucidation of the question at successive International Conferences— The abolition of quarantine.

Part of the quarantine system — Basis of the substituted system — Development of organisation necessary for the extension of its application — Advantages of notification — Objections raised — Compulsion — Systems, single and dual — Notice and certificate — Diseases included in the English Act — Other diseases, erysipelas, measles, etc. — Conditions necessary for efficient working — Practical objects in view — Registration, local, county, national, international — Concealment — Compulsory examination.

CHAPTER III.

Exclusion and seclusion — Measures of Exclusion — Schools and institutions — Dwelling-houses — General and special hospitals — Aggregation of infectious cases — Effects upon mortality — The object of hospitals for infectious diseases — Temporary and permanent hospitals — Pavilions — Rectangular wards — Circular wards — Ambulance — Metropolitan system — Practical value of isolation in hospital, and its complement.

CHAPTER IV.

Disinfection in its popular sense — Purification — Disinfectants — Deodorants — Antiseptics — Preservatives — Germicides— The destruction of infection — Mechanical means — physical and chemical agents — Cleansing and exposure to the elements — Subjection to heat— Hot air, dry and moist — Boiling water — Steam — Effects of pressure — Steam disinfecting chambers — Current steam— High-pressure steam — Method of working — Chemical agents— Solutions, mercuric chloride, carbolic acid — Gases, chlorine, iodine, bromine, sulphurous and nitric acids — Disinfection of interiors — Excreta, secreta, and discharges—Rags and dressings — Linen and other washable objects — Un washable objects — Fumigation of air — Cleansing

CONTENTS. IX

TAGE

of surfaces — Temporary shelter for occupants of single rooms — Disinfection of apparel and person — A complete disinfecting station.

Chapter V

Inoculation of small-pox — Its introduction and results — Vaccination— Small-pox mortality before and since vaccination — Recent testimony of semi-civilised communities — Influence of vaccination — Upon the epidemic cycle of small-pox — Upon the age-mortality of small-pox— Influence of the quality of vaccination — Effects of re-vaccination — Advantages of calflymph — The objects of compulsion — Inoculation and vaccination in other human diseases — Preventive and curative inoculations— Yellow fever and cholera — Rabies and tuberculosis — Inoculations of animals.

Chapter VI

Importation — Prohibition — Quarantine— Slaughter — Declaration — Notification — Isolation — Infected place, circle, and area — Cordons — Slaughter and compensation — Destruction of infected carcases and objects — Disinfection — Animal products — Milk — Meat — Partial and total destruction — Inspection of animals and meat — Disinfection of meat — Entozoal affections • Raw foods — General precautions against entozoal diseases.

Chapter I

Development of the dwelling and urban conditions — Sources of disease and conditions of health — Sanitation — Water — Advantages and requirements of a public supply — Quantity, quality, pressure, and constancy— Sewage disposal — Single and separate systems of sewerage — Domestic precautions under both systems — Refuse disposal — Smoke abatement — Crowding together of persons, rooms, houses, blocks, and quarters — Light and air— Cubic space — External surroundings — A standard of light and air called for.

Chapter II. The Dwelling-room . . . . . .294

Dimensions— Height — Lighting — The window, area and situation — Depth— Angle of light — Width — Heating — The fire-place — Advantages and disadvantages of the open grate — Solid coal and coal-gas as fuel — Vitiation of air — Ventilation — Outlets and inlets — Distribution and diffusion — Simple ventilating appliances — Dependency upon occupants — The door — Purity of the air of the house— Ventilating, heating and cooking range — Perflation — Relative positions of window and door — The staircases and passages.

X CONTENTS.

FAGE

Chapter III

Site and soil— Impervious basement — Damp-proof course — External dry area— Imperviousness and impermeability of walls, roofs, ceilings, floors, and interior surfaces — Non-conducting materials — Aspect — Angle of light — Obstruction — External perflation— Exclusive space — Corner houses — Yards — Physical effects of pipes — Water-service — Cisterns and flush-tanks — Constant high pressure — Reserve — Precautions — Drainage- -Trapping — Disconnection — ■ Ventilation — Sewer-to-house drain- House drain — Soil pipe — Water-closets — Waste pipes — Rain-water pipes— Overflow pipes.

Chapter IV. Classes Of Dwellings ...... 325

Movable and fixed dwellings — Made-down houses -Houses let in separate dwellings— Houses let in separate lodgings — Common lodging-houses — Underground dwellings — One- and two-roomed dwellings — Through ventilation or perflation, and its modus operandi — Detached, semi-detached, four-group, and back-toback houses — Houses with one front only — Stable-dwellings — Lofts— Causes rendering dwelling-houses unfit for habitation — Influence of the value of land — Single rooms, single dwellings, and parts of dwelling-houses unfit — Specially constructed dwelling-houses — Model common lodging-houses — Collective dwelling-houses — Requirements of blocks of dwellings — Exclusion of gaseous pollution — Provision for ventilation, lighting, and perflation — Inadequacy of Building Acts.

Retrospect and summary — Education in personal and public health — Popular and expert instruction — Administrative bodies and executive officers — Communicable disease — Parasitology and questions awaiting solution — Prophylaxis — Greater uniformity of action — Abolition of quarantine— Port defence — Inland defence— General compulsory notification of infectious diseases — Suppletory nomenclature to scheduled diseases — Registration of infectious diseases— Local, county, national, and international registration, compilation, and exchange — Compulsory clinical instruction in infectious diseases — Increased provision of isolation hospitals — Greater restrictions upon exposure, and neglect of isolation— Minor infectious diseases — Infectious mortuaries — Temporary shelters — Disinfection — Vaccination — Necessary reforms — Communicable diseases of animals— Public abattoirs — Licensed milk stores— Dwelling-houses— Greater restrictions upon the obstruction of light and air required — Greater power to deal with single rooms and parts of houses — Regulations requisite for stable dwellings — By-laws improved for common lodging-houses — Building Acts for new dwelling-houses supplemented—Original construction and subsequent maintenance distinct considerations— Both important to the health officer — Educated public opinion the basis of advance in public health.

INDEX 357

Preface

INCREASING knowledge of the inimical factors acting upon the health of communities, and the application of practical measures for the prevention of disease, have given rise in recent years to many interesting problems that are passing through various stages of solution. In this volume an attempt has been made to bring to a focus some of the essential points in evolution, environment, parasitism, prophylaxis, and sanitation, bearing upon the preservation of public health. The scope of the subject is necessarily wide, but the central point round which its treatment has gathered is the aspect presented to a health officer in reviewing the influences operating around him. I am only too conscious that my work falls short of completeness, but the reader will realise that the complexity of the task conveys also an idea of the ramifications and intricacy of the problems dealt with by workers in the public health service.

Statistics have been avoided as much as possible ; my endeavour has rather been to cast a reflective and suggestive line of thought into the volume, perhaps xii PREFACE.

at the sacrifice of some detail in parts, on account of the gaps that observation and research have not yet filled. With the materials available, it became apparent that a clearer idea of the future drift of manjr public health questions might be obtained by tracing the phases through which they have passed to emerge into their present light. It may be objected that, in reference to sanitation, only the particular points relating to the dwelling have been touched upon, but engineering questions have been purposely avoided with the object of throwing into relief more elementary principles, especially in reference to light and air, which are so persistently neglected, but which are such important factors in influencing the health of large urban populations.

I am greatly indebted to Mr. Havelock Ellis for much valuable advice given at appropriate moments in the course of the manuscript, and I must express my thanks to Dr. John Glaister for carefully reading and annotating the proof-sheets with suggestions whilst the book was going through the press.

J. F. J. S.

40 Camden Square, N.W., September 1892.

PUBLIC HEALTH PROBLEMS.

Introduction

"For life is not to live, but to be well," said Martial, who lived in the first century, when Rome was in the full enjoyment of her aqueducts, baths, gymnasia, public latrines, cloacse, pavements, scavenging systems, and sanitary edicts, so graphically portrayed by Sir John Simon in his E?iglish Sanitary Institutions. Life has been defined by Herbert Spencer, in his Principles of Biology, as "the continuous adjustment of internal relations to external relations," and health is the expression of this response to environment.

Health being, therefore, a relative term, its measure varies, within limits, according to the age, sex, family, and race of the individual, and the physical, chemical, and biological influences surrounding him ; and it is maintained by the utilisation of beneficial influences, and by the avoidance, deflection, or suppression of inimical influences. The standard of individual health, when estimated by the physical or intellectual forces displayed, varies more than when measured by the anatomical and physiological gauge, the absence of disease and of disproportion, ascertainable by instruments of precision, being assumed as a negative proof of health. Positive measurements of the proportions of certain structures and functions are already adopted in selecting candidates for the services, for life insurance, and for other special purposes ; the anthropometric investigations of Francis Galton and others may be trusted to

i supply us ultimately with definite standards of anatomical structure, and physiological function and adaptability.

Like the health of the individual, the health of the community is mainly measured by negative results, the minimum mortality and morbidity ; that is, the fewer deaths and the fewer sufferers from disease per thousand per annum of the population, sex, age, and other factors being equal, the healthier the community; the positive measurements being the duration of life, ascertained by constructing a life table of the relative numbers, ages, and sexes of those living and dying during a certain period, and the number of working days available, ascertained by comparing the number, age, and sex of the sick, the nature and duration of sickness, with the number, age, and sex of the population. Statistics of sickness are drawn for the army and navy, in which services accurate records are kept, but the necessary data are not available for civil populations. As in the case of the individual, the physical and intellectual powers developed by communities vary more than the vitality, and are measured by a different class of statistics.

An early allusion to statistics follows from the fact that it is only by this means that standards of structure and function, health and disease, life and death, can become established, and that personal as well as public hygiene is guided by their conclusions.

Statistics are based upon the census enumeration, within a given area, of the number, age, sex, and occupation of the population, and in proportion to these the vitality, mortality, and morbidity are calculated. The vitality is computed upon the marriages, births, deaths, and sickness, the mortality upon the deaths and causes of death, the morbidity upon the nature and duration of sickness, which includes both injury and disease, and upon the number, age, and sex in each class.

These data are obtained from the registers of marriages, births, deaths, and sickness, the causes of death and nature of sickness being obtained by medical diagnosis, and upon the skill of an educated medical profession ultimately depends the accuracy of all mortality and morbidity statistics. It is very apt to be treated lightly, or to be overlooked, that the prevention of disease amongst a people is fundamentally

INTRODUCTION. 3

dependent upon an educated medical profession, trained both in practice and experiment.

Experiment as well as statistics form the groundwork of hygiene in all its ramifications. The science, which was born with the rudiments of civilisation, has increased in growth pari passu with scientific and social development, and has put forth many branches. Personal and public health meet at many points in the various branches of hygiene, domestic, scholastic, and occupational, naval, military, and civil, urban and rural, national and international, built upon many sciences by many professions, but divisible broadly into the causation of disease and its prevention. Etiology, or the causation of disease, is the offspring of medicine, and the indications for disease prevention have greatly stimulated the study of the collateral sciences.

The efforts of the individual in the preservation of health are limited, and the advice of Pythagoras to find out that course in life which is best, and then habit will render it the most delightful, is followed with difficulty even by the most favoured individual. How much fewer, then, must be the opportunities possessed by the least favoured amidst the stress and turmoil of providing for existence under modern industrial conditions? The training of the young in bodily and mental exercises and healthy habits, coincidentally with education in the accumulated experience and knowledge of preceding generations, rapidly bridges over the wide gulf of time that unaided individual effort would require to find the best course.

The slow processes of nature, if capable of coping with segregated rural conditions, are outstripped and overtaxed by the aggregation of communities in limited areas, every individual of which is dependent upon his neighbour for the condition of his surroundings. Not only does communicable disease readily settle and extend in concentrated populations so as to necessitate public measures of prophylaxis, but conditions are established that cause deterioration and degeneration, rendering the inhabitants a ready prey to other diseases, and requiring measures of sanitation for the maintenance of health. The science of public health is not content with the suppression of communicable disease, but aims rather at securing those conditions of the health of a community that shall enable it to display the utmost activity of its physical and intellectual energies. It must be admitted that this conception of the task is somewhat extensive; nevertheless, however far it falls short of its realisation, it is a labour worthy of the many workers contributing from various sources to its accomplishment.

Public health problems, formulated by medical, biological, and scientific evidence, involve legislative, administrative, and social changes of the widest extent in their solution. The social element is a factor to be largely reckoned with. State remedies cannot be applied in advance of public opinion, and this is slow to move. The education of a vast community is perhaps the most difficult task that falls to sanitarians. Persuading the unscientific mind to reason logically, even after possession of the facts, is not a light task. To rouse it to take action, even when convinced, and to overcome prejudice, requires a prodigious effort. Cremation makes but slow advance, scarcely recognisable. Muzzling of dogs, as a measure for the prevention of hydrophobia, raises the antipathy of those who are concerned more for the canine species than for their fellow-men. The opposition to experimentation upon animals by man, whilst nature herself is hourly experimenting upon human beings ; the opposition to vaccination mainly because it is compulsory, and in disease attributed to its influence post hoc is not distinguished from propter hoc ; the opposition to the suppression of sexual disease in garrison towns because of the failure to appreciate the fact that the State practically endeavours, and fails, to impose celibacy upon a large body of men, without attaching the moral restrictions of a priestly calling, or imposing the physical barrier of emasculation : — these things must surely bring to the lips of the legislator the words of Galileo, E pure si muove.

Modern pathology seeks far back for causes of disease. Heredity and the effects of internal influences upon the health of the individual and the community have their place, and not an unimportant one, in public health problems. External influences are traced a step further than was considered necessary in former years ; their effects upon other forms of organic life which react upon man must be included in the hygiene of the future.

INTRODUCTION. 5

But the special feature that is rapidly changing the aspect of pathology is the study of the communicable entities of contagia, and all future work in infectious or contagious diseases will be based upon a parasitic theory. Our knowledge of these diseases is increasing so rapidly in exactitude, and extending so widely in scope, that it is difficult even for experts to keep pace with the advances of scientific research all along the line of micro-biology.

Since the discovery by Pasteur of the method of growing bacilli outside the living body, followed by Koch's discovery in 1880 of the mode of separating species of microbes by cultivation on the surface of solid media, and again followed by his discovery in 1882 of the tubercle bacillus, and the complete proof of the parasitic nature of an infectious disease in man, the progress of bacteriology has advanced, comparatively speaking, in a geometrical ratio, and a host of observers have been at work recording observations upon communicable diseases among plants, animals, and man.1

The burning questions of the hour in general biology are the variations that organisms, complex and simple, are subject to under changed environment, and the transience or permanence of these variations. The equally important problems in micro-biology are the attenuation and intensification of the virulence of micro-organisms under varying conditions, and the immunity naturally existing, acquired, or artificially produced, against diseases in man and animals due to such microbes. The outcome of researches in this new branch of the science of micro pathology, by explaining adaptation and variation, will in the future throw a clearer light upon the greater issue of heredity.

The growth of the organisation that in this country has enabled the researches of pathology to find practical application in preventing the spread of disease dates back to the commencement of civil registration in 1837, and the work of Farr, Chadwick, and Simon in statistics, investigations,

1 In this direction human pathology is being supplemented by the publications of the Veterinary and Intelligence Departments of the Board of Agriculture, that record the origin, prevalence, and extension of contagious diseases among animals and of destructive parasites among plants.

and administrative measures, continued by the Registrar- General's office, the Local Government Board, and the Local Sanitary Authorities to this day. Thus the revelations of more recent pathology find a complete organisation at hand for the practical application of measures indicated by scientific research. The value of this is expressed by the progressive fall in the mortality of this country.

The methods of prevention, and perfection in the measures adopted, are following closely in the footsteps of research. It is becoming a matter of past history how plague and, to a certain extent, cholera have been driven back out of Europe, and other infectious diseases promise to follow in their wake. Relapsing fever and typhus epidemics are less frequent visitants than formerly, and not only is the mortality from other zymotic diseases being reduced, but the general mortality of this country, and especially that of the great centres, has fallen in recent years in a marked degree. This is acknowledged to be due to the organisation of the public health service of this country, and to the prophylactic and sanitary measures indicated and executed.

A still greater work has been, and is being, accomplished by this service in the powerful educational effect, not upon the young so much as upon the adult population, resulting from the continuous work of the large body of health officers and sanitary authorities.

Public Health in its later developments has realised that it could never hope to satisfy its aspirations by the prolongation of the existence of the more favoured members of the community. Its greatest justification lies in increasing the health and power of the workers, and especially of the industrial classes, those less able to help themselves. It is distinctly socialistic in its tendencies as distinguished from individualistic ; it is, in the widest sense, humanitarian. The nation that makes some effort to assist a neighbouring nation in minimising the evils of famine and war, even at some sacrifice of patriotism, may reap the reward by averting an epidemic, inhibiting it at its birthplace, just as a smaller community may protect one adjoining, or an individual his neighbour.

It is well-nigh impossible to form an adequate conception of the problems involved in defending the health of a

INTRODUCTION. 7

population without having in mind the various elements that undermine and overpower it. First amongst these, and introduced at birth, are the questions that hover around heredity and the potential effects of intrinsic characteristics upon the future of the being. The physical, chemical, and biological influences external to the organism follow in natural sequence. But in considering the effects of these influences, when they produce injurious or prejudicial results, the order of priority is reversed by practical social conditions, communicable diseases due to biological causes claim the first attention, and prophylactic measures for their suppression take precedence in the health administration of a community. Injurious chemical and physical influences, though not less important, demand more time for their rectification by requisite sanitary measures. The two classes of remedial effort act from opposite standpoints, the former attacking disease, as it were from the rear, by suppressing it in its track, the latter advancing towards humanity equipped with a knowledge of the requirements of healthy existence, and securing their provision.

Such is an outline of the ideas to be conveyed within the limits of this volume, but as to sanitation, it will only be possible to touch upon the main points as they present themselves amongst an urban population, especially in reference to the dwelling ; nothing short of an encyclopaedic work could encompass more.

PABT I.

THE EFFECTS OF INTERNAL AND EXTERNAL INFLUENCES UPON HEALTH.

Chapter I. Heredity

The Darwinian theory of natural selection has given prominence to two schools of evolutionists, the one attributing evolution solely to selection, and the other, whilst not denying the effects of selection, valuing — perhaps over-valuing — the effects of heredity. The difference of opinion may in a measure arise from difference in the methods of treating the subject, partly also because of the varied limits of the meaning attached to terms, but greatly from the present insufficiency of our knowledge. The one school is mainly deductive, and the other inductive in method. Weismann may be taken to represent the former, and Herbert Spencer the latter. Darwin himself retained the older Lamarckian theory of heredity to explain those facts of evolution not explicable by selection.

The theory of natural selection is based upon certain fundamental principles, the power of rapid multiplication of organisms in geometrical progression, the struggle for existence which follows, and the survival of those organisms whose variation is fittest to cope with their environment, no two organisms being identically similar, although bearing a general likeness to their parents and to each other.

So far there is no divergence of opinion, but the origin of variation is the rock on which the two schools split. The deductive school maintains that the sole source of the variation is the intermingling of the varied sexual elements of the parents, and that characters acquired by the individual are not hereditary. The inductive school, proceeding from the opposite end of the pole, argues that the influences of environment act upon structure through function, that function is modified by adaptation to environment, and that "adaptive change of function is the primary and ever-acting cause of that change of structure which constitutes variation."

It is as well to be clear as to the meaning of the terms "variation" and "adaptation." It is admitted that the type is hereditary, and that the variation which by natural selection survives is hereditable — in short, that variation is a congenital modification, existing or potential. The term "adaptation" is not used in an equally definite sense. There is an adaptation taking place in the individual (Spencer's direct equilibration), and adaptation of the species (Spencer's indirect equilibration, and he points out that this is identical with that process of survival of the fittest which Darwin named "natural selection"). Weismann only recognises the term as applicable to the species, against which Eimer protests. Therefore, in referring to the opinions of the inductive school, it is advisable to understand the term as mainly applying to individual adaptation.