Page:George Archdall Reid 1896 The present evolution of man.djvu/226

consequence the whole organism is) brought into completer harmony with an environment that has been rendered more complex by the presence of pathogenic microbes—is in full accordance with the theory of evolution set forth in the preceding pages. The variations which the phagocytes undergo, when they acquire powers of attacking pathogenic micro-organisms, which were before invulnerable to them, are strictly comparable to the variations undergone by other, kinds of cells under appropriate stimulation; for example, to the variations undergone by skin-cells when subjected to oft-repeated heat or friction, as in the blacksmith's hand, whereby they are able the better to resist heat and friction; to the variations undergone by muscle-cells, when, as a result of training, they acquire the power of contracting long and vigorously; to the variations which the eye-cells undergo in an educated man, who without inconvenience can for hours scrutinize small print, a proceeding that would render the eyes of an uneducated man dim and painful in a few minutes; to the variations undergone by the cells of various other tissues, whereby toleration is established against nicotine, arsenic, &c.; in short, to the variations undergone by this or that kind of cell in every variation by means of which the organism is brought into harmony with the environment, for any such variation is after all nothing other than a variation, quantitative or qualitative, or both, of certain of its component cells. Moreover, the theory incidentally affords a valid explanation of the otherwise inscrutable fact, that acquired immunity against any zymotic disease protects against that disease only and against no other. À priori, we might suppose that the acquired powers which enable the phagocytes to attack and overcome the microorganisms of any disease, would be available, in some degree at least, against the micro-organisms of all