Page:George Archdall Reid 1896 The present evolution of man.djvu/242

to say impossible, that this theory correctly explains the phenomenon, because, since the toxins of every disease differ from those of every other disease, as is proved by the difference of their effects on the organism, this would mean that the host is capable of elaborating separate anti-toxins, highly complex chemical substances, exactly antagonistic to the different toxins of all the separate zymotic diseases to which he is susceptible, and against which he is able to acquire immunity; a thing wholly incredible, especially when we remember that' these anti-toxins may, if the theory is true, be manufactured by individuals whose ancestry can have had no experience of the disease, and in whose race therefore there can have been no evolution of the power of such manufacture; for instance, the horse is resistant to diphtheria, and is able to acquire immunity against it, yet we have no reason to suppose that the equine race has in past ages been afflicted by the disease; again, the natives of Australia and the islands of the Mid Pacific are able, though less able than the inhabitants of the Old World, to acquire immunity against such Old World diseases as measles, scarlatina, small-pox, &c., yet, as we know, these diseases are new to their races. Moreover this theory of immunity by the production of anti-toxins is unnecessary, since the theory of immunity by fit variations sufficiently explains the phenomena. We can well believe that in the horse and in the Samoan the amoeboid leucocytes have retained the phagocytic functions of their remote amoeboid unicellular ancestors, and that in these cells there has been an evolution of the power of varying in a fit direction in response to stimulation, whereby they are able to react to stimulation from the toxins, even though these toxins are strange to them and to their ancestry, just as other cells of the body are able to vary in a fit direction when stimulated, even when the stimulation is strange, this being the especial