Popular Science Monthly/Volume 27/October 1885/Tissue-Selection in the Genesis of Disease

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IT has occurred to the writer that the adoption of the germ theory of disease necessarily involves the application of the theory of evolution, and that here may be found a means of accounting for the genesis of the various forms of disease. Germs are living matter; they must therefore be under the influence of those laws and forces which condition all living matter. The most important of these, or the one that most interests us in the present connection, is the law of natural selection; and it must be that germs, in common with other forms of life, are under the influence of this law in some shape. Natural selection is a general term which embraces all other modes of selection, or provision for the "survival of the fittest." Among these is sexual selection, and, taking a broad view, we may also include selection by man. In order, then, to the attainment of greater exactness, may we not give a name to that form of natural selection which has been potent in bringing about the variations in the characteristics of those germs to which the differences in the forms of disease are due? Such a term may be found, I would suggest, in "tissue-selection," as indicating the special means whereby the constitution of the germs has been modified. The actual origin of the bodies which have received the names of germs is not at present determinable, and to say that they do not originate de novo, in decaying matter or elsewhere, is only to reaffirm the axiom now pretty generally admitted, Omne vivum e vivo. The fact that meets us here is that these germs, call them bacilli, vibriones, bacteria, or what we will, are met with almost universally in the atmosphere that we breathe. It is with the "why and the wherefore" of their existence that we are concerned. In looking for this, it will be necessary to consider the facts of the life-history of these germs, and to try to discover how they have been and are influenced by their surrounding conditions. We find that the conditions favorable to the full vital activity of these germs are a moderate temperature, moisture, and a resting-place or nidus in some organic matter whose chemical constitution enables it to afford the pabulum necessary for the maintenance of their existence. On the other hand, the influences antagonistic to their well-being are excessive heat or cold, the action of certain chemical bodies, and a condition of dryness. The last is certainly prejudicial, as it seems to hinder their full vital activity. At the same time it can not be considered absolutely obnoxious, as it is the means which favors their locomotion in the atmosphere. Putting aside, for the mean while, the thermometric and atmospheric conditions, we shall see that the condition which most concerns us is that of a favorable nidus. A favorable nidus is one in which the germ is enabled to carry on its full vital functions, and to propagate its kind. It is therefore manifest that the constitution of the nidus must be free from elements antagonistic to such vital functions. It must not be too hot, or too cold, or too dry, and its chemical constitution must be favorable. Experiment has proved that oxygen in excess is deleterious to these organisms; its absence, entire or partial, will then be one of the requisites in this chemical constitution of the nidus. Is not such nidus best found in decaying or degenerating animal tissue? Where there is full vital activity in any animal tissue, the blood which nourishes it keeps it duly supplied with oxygen. In healthy tissue, then, we have a condition unfavorable; but when from some cause the nutrition of the tissue is interfered with and a condition of degeneration is induced, this antagonistic element is removed or at least diminished, and the tissue affords the nidus favorable to the vital phenomena of the germs. These vital phenomena are perhaps best studied in the analogous case of the spores of yeast (torula cerevisiæ). When this, which is to all intents and purposes a germ, is placed in favorable circumstances, its activity commences, it rapidly multiplies and gives rise to changes in the surrounding material. In this case we call the process "fermentation." A germ, bacillus, bacterion, or vibrio, when placed in relation to tissue which affords a favorable nidus, assumes its full vital activity; it multiplies and gives rise to changes in the tissue with which it is in contact. These changes we call "inflammation." In fact, it would appear that these germs in one sense fulfill the part of Nature's scavengers, and by setting up inflammatory changes in degenerate tissues lead to their removal. Be this as it may, the diseases to which the germs give rise are all more or less of an inflammatory nature.

Thus, then, it would seem that one of the chief vital functions of these germs is to excite an inflammatory process in degenerate tissue. Is it not conceivable that germs may have existed, or even do still exist now, whose function is strictly limited to action on degenerate tissue?—that this may perhaps have been the limit strictly assigned to them? Let us suppose this to be the case, and see how, from this limited condition, germs have acquired power to overstep these limits, and thus to give rise to the protean aspects of disease that we now meet with. What has caused variation in the animal world but the influences of surrounding circumstances? In the relationship of the germs and the degenerate tissue, it is plain that in one sense the germs are the active, and the tissue the passive, elements. But, looked at from the tissue point of view, it will also appear that this passive condition possesses considerable indirect influence on the germs; that, indeed, "passive" is hardly the word to express the action which must largely modify their constitution. It would be impossible for the germs to live, to grow, to multiply on a certain tissue without becoming imbued with certain characteristics of that tissue. Hence a difference, a variation, between the parent germs and their offspring.

Now, the one characteristic of the tissue which most strikes our attention is its degeneracy, and this degeneracy must exert its influence on the organisms which depend upon it for existence, so that in the thus derived organisms we have the germ vitality and function, plus a certain amount of degenerate tissue characteristic. Germs thus modified and brought into contact with tissue of the same kind, though less degenerate, will, in virtue of this constitutional modification, stand a better chance of establishment thereon, at the same time adding to the degenerate condition with which they meet in the new tissue the characteristics of the degeneracy of tissue they have left. The effects thus produced will be more acute, the constitution of the germs further modified, and their power increased. Hence it is possible to conceive that if by any chance a condition of tissue which could be called perfectly healthy was to be anywhere met with, even this tissue might in time become subject to the influence of these organisms. Of course tissues may be more or less susceptible to their influence, more or less healthy, but it is more than doubtful whether it can be positively said of any one tissue that it is absolutely healthy, any more than it can be said of the individual man or woman. It is therefore possible to conceive that at some time or other there has existed only one kind of germ, that variations from this one type have arisen in consequence of the modification wrought upon different individuals by their chance falling upon this or that degenerate tissue. Variation must lead to specialization, and finally we find diseases all dependent upon the action of germs, as different from one another as one species of animal differs from another.

But there is another direction in which these germs must have been modified fully to account for the differences we now see. Germs may still exist which have only the power of exciting a simple inflammation in any degenerate tissue. Others, a step more advanced, are found, whose action is more potent, which have attractions for the one special tissue in which they have been bred; while others are capable of exciting the special form of inflammation in which they have had their origin, in tissues various in structure and composition. This power or, rather, increase of power—in other words, further variation in constitution—thus displayed is only to be accounted for by the supposition that its acquisition is secondary to the act of establishment, and that it is brought about by means of the blood itself receiving some of the germs and conveying them to some other tissue, on which, in virtue of its degeneracy, and possibly of a further modification, they have themselves received from the blood, they are enabled to effect a settlement. It must also be remembered that their modification, which enables them to select a tissue for their primary and more virulent action, does not deprive them of their original power of attacking degenerate tissue. It is simply an additional power, rendering them more potent when Drought into contact with tissue possessing certain characteristics. These characteristics of the tissue originally attacked may determine the character of the action of the germs on other tissues, and thus enable us to recognize a relationship in the morbid influences at work in different tissue—as, for example, in syphilitic disease. If it is a fact that the tissue originally attacked modifies the constitution of the germs, it is surely more reasonable to suppose that the special characteristics thus impressed upon the germs are those which give the family likeness to the descendants of these germs, rather than to suppose that this morbid family likeness exists in the germs before ever they have been brought into contact with any form of degenerate tissue. In other words, may not a bacterion, pure and simple, by being brought into contact with some form of degenerate tissue, acquire or give rise to other bacteria which possess certain characteristics which we recognize as being those of special disease, and that those characteristics enable them to excite a similar form of disease in the new nidus? It is also conceivable that as different diseases are more or less wide-spread or of older standing, so the germs given off from tissues affected by these diseases will have more and more power of affecting tissues less and less degenerate, even to the point of a possible condition of actual health. So, for example, the germs of scarlet fever or measles have actually acquired great power over the tissue which they primarily affect; whereas in the case of tuberculosis it is more than doubtful whether the actual presence of a tubercular condition of degeneration in the tissue, to a definite extent, is not necessary before the germs can find a suitable nidus in that tissue. Of course, germs derived from a tubercular origin will have more influence over tissue which is not so much affected than germs which have not such tubercular origin. It is, in fact, simply a matter of degree of degeneracy. May it not be this which enables persons free from tubercular taint to brave the assaults of germs be they ever so virulent?

The above are only one or two examples showing the different powers possessed by germs derived from the different forms of disease. The list of examples might be almost indefinitely prolonged. But the result would only be to show that different diseases have different powers of affecting the animal body, a fact already well known. It is the design of this paper to show that possibly these powers have been acquired by the action of some form of natural selection, here propositionally called "tissue-selection," on the germs which inhabit the air we breathe and the water we drink, such action in the course of long "ages having given rise to germs possessing special powers, and being now apparently unrelated to each other in any way. The intermediate steps between a simple germ and such a highly modified one, as, for instance, the variolous or tubercular germ, may be difficult to trace, for as we at present see these germs it is as though we were looking at a picture or photograph of a family tree in which the greater part of the trunk and larger branches have become obliterated, having only the terminal twigs visible, with no apparent connection between them.—Lancet.