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THE PRESENT EVOLUTION OF MAN—PHYSICAL
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other diseases, but experience proves that this is not to any extent the case. An attack of measles, for instance, does not at all protect against scarlatina. But when we learn that acquired immunity against any disease is due to a power acquired by the phagocytes of withstanding the toxins of that disease, and when we remember, in this connection, that the toxins of any disease must be different from the toxins of every other disease, since their effects on the organism are so different, we are able to understand why acquired immunity against any one disease holds good against no other.
