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Taking, then, this wide view of the scope of public hygiene, the subject of public health and preventable disease may be briefly discussed under the following sections
I. Hereditary Influence.
II. Causes of Deterioration and Disease.
SECTION I. HEREDITARY INFLUENCE.
Although there are many biologists who do not accept the Darwinian theory of "Pangenesis" in its entirety, there are few amongst them who dispute the influence of heredity which it serves so fully to explain. As regards the individual, the term heredity is the expression of the fact that a man is wholly built up of his own and ancestral peculiarities, and so far as these are concerned, it matters little or nothing what were the characteristics of the early progenitors of his race. In other words, the accumulation of individual variations through recent descent has a far greater influence upon a man's bodily and mental constitution than the unchanged gifts of a remote ancestry, so much so, indeed, that these latter may be regarded as a vanishing quantity. This result of the Darwinian theory has been conclusively demonstrated by Mr. Galton in his work on Hereditary Genius, and it is especially valuable as affording a clear and succinct conception of the influence of heredity in all cases in which individual variations lie well within the limits of stability of a race.
According to this view, the progeny is invariably moulded by the characteristics of its more recent ances
try, and by those of the parents more than by those of its grand-parents, and so on backwards in a constantly decreasing ratio. It also tends to show that personal characteristics or peculiarities, however much they may apparently differ from those of immediate progenitors, are not so independent as might at first sight be supposed; that they are, in reality, modified "segregations" of what already existed, either partly or wholly, in a latent condition.
But without pursuing the subject too far into the regions of controversy, it will suffice for the present purpose to adduce some of the more important opinions which are entertained by leading biologists concerning the influence of heredity, alike on body and mind, in health and disease. These may be summarised as follows:
1. The influence of both parents on the bodily constitution of the offspring is manifested in personal resemblances, such as, stature, similarity of features, walk, gesture, colour of hair, etc. Some of the children may bear a greater resemblance to the father, others to the mother; but it is rare to meet with any instances in which some distinctive characteristics of both parents cannot be traced.
2. The influence of the other more immediate progenitors on the bodily constitution of the offspring is manifested by the resemblances which constitute the phenomenon known as atavism, which may be explained in this way:-A man, for example, does not inherit all the characteristics of either his father or his mother, and of those which he does inherit, only some are developed, whilst others remain latent, and are probably developed in a brother or sister. His son, how
ever, may in turn inherit the same characteristics, but with this difference that those which were latent in the father become fully developed in him, so that he comes to bear a stronger resemblance to a grand-parent or some other relative, as an uncle or aunt, than to his father or mother. (A Physician's Problems, by Dr. Elam.)
3. The influence of race, or special type, in heredity, is manifested by the constancy of averages, under tolerably constant conditions, from generation to generation, and this not only as regards the whole body and its various component parts, but also as regards all the facts which are comprised in the wide range of social and vital statistics.
4. Deviations from these averages or from the normal type, although they are transmissible, cannot transcend certain limits. Thus, as regards size, the giant and dwarf form the extreme links of the chain; and hence, in the procreation of individuals representing these deviations, the tendency to revert to the normal type is invariably manifested in the offspring.
5. As all forms of deterioration or disease may be regarded as deviations, or perverted life-processes, they are likewise subject to limitation in transmission, and there is the same tendency exhibited to revert to normal type under improved conditions. Thus, all chronic diseases appear to be transmissible, either as a morbid tendency or in their general form, such diseased heritage being well exemplified in the case of gout, scrofula, phthisis, syphilis, and insanity; but by adopting suitable measures, the disease may be finally eradicated from the family, or the morbid tendency be overcome. It has also to be remembered that a hereditary disease or a morbid tendency may remain latent,
like any other characteristic, for one or two generations, and become developed in the next.
6. Mental and moral qualities, if indeed they can. be separated, are subject to the same law of heredity as other personal characteristics, with this important addition, that any vicious habit or tendency in the parents becomes, as a rule, intensified in some form or other in the offspring. With regard to mental capacity, Mr. Galton has clearly demonstrated, in the work already referred to," that a man's natural abilities are derived from inheritance, under exactly the same limitations as are the form and physical features of the whole organic world" and Herbert Spencer, Morel, Dr. Maudsley, and many others, uphold that a man's mental incapacity is similarly conditioned,-that perversion or absence of the moral sense is, in effect, as much a portion of some men's inheritance as their height or weight. In no class of persons is the truth of this doctrine better exemplified than amongst habitual criminals;-their strong impulses and feeble wills, their vicious propensities and absence of moral sense, which constitute the fate of their heritage, render them more or less irresponsible members of society.
7. Any particular characteristic, especially if it be of the nature of a deterioration or taint, when common to both parents, is liable to be intensified in the offspring. It is on this account that marriages between blood-relations are inadvisable, inasmuch as latent morbid tendencies, should they form part of the organic patrimony of the family, are almost certain to become developed in the children.
Such being the influence of heredity on man's physical, mental, and moral being, what, it may be
asked, are its bearings on public health? Briefly these that each generation has enormous power over the well-being of those that follow; that acquired habits, whether for good or evil, may become more or less permanent in a race, the good being slowly developed and with difficulty retained, the evil readily implanted and with difficulty eradicated. It shows also that deterioration, however produced, as it affects families, may affect communities, in an ever-widening circle, until a whole race may become degenerate and disappear from amongst nations.
SECTION II-CAUSES OF DETERIORATION AND DISEASE.
These may be divided into two classes-namely, social and material. As the material causes will be more or less fully discussed in succeeding chapters, the mere enumeration of the more important of them will suffice here, such as impure air, impure water, insufficient or unwholesome food, dampness of soil, deficiency of warmth, etc. The removal of these causes is the principal aim of practical hygiene as enforced by legislative enactments. The social causes of deterioration and disease, on the other hand, are little, if at all, controlled by State interference; and hence their removal, as far as possible, must depend mainly on individual or combined efforts dictated by a sense of duty, which may be either egoistic or philanthropic, as the case may be. It is here that the effects of education, whether imparted in the family circle and school, or from the pulpit and platform, or by the public press, will be tried and tested.
In a country such as this the social causes of