This induced immunity from further attacks has received in the hands of Metschnikoff a curious and very interesting explanation. He has shown that an army of small cells, called phagocytes, which wander through the blood and tissues, are able to attack the microbes of disease, and that after a struggle they are able in many cases to kill these voracious invaders. In so doing, however, the weaker phagocytes succumb to the struggle, while those which are left alive within the body of the convalescent patient possess the power of resisting and destroying the particular microbe which had undertaken the invasion. These resisting phagocytes, selected from the rest, together with their descendants, who share their resisting qualities, are able to prevent fresh inroads of the same enemy. We need not, therefore, assume that this acquired immunity, the sole relic of the attack, indicates any change in the ordinary muscle or brain cells of the body, or that the reproductive cells are in any way altered, for the immunity is due only to a change in the phagocytic army. The germ cells in almost every case get off scot-free, and there is nothing in the organisation of a child to indicate whether or not his father or mother suffered from measles, or scarlet fever. It might at first sight be urged, in opposition to this fact, that, although we cannot recognise the child of a man who suffered from measles or scarlet fever by any visible sign, yet the child is in some way different, inasmuch as he is to some extent immune to those diseases. In favour of this belief, the many instances in which a fever has been brought to a country never before accessible to the germ (for instance, the introduction of measles and small-pox to newly discovered America, where fearful ravages were caused thereby), may be brought up as evidence to prove that those habitually living among the germs must have become immune and have transmitted this immunity to their progeny. Again, the black population of Sierra Leone have only a mortality of 24 per cent. from malaria, while the mortality of the white settlers is 47 per cent.;[12] and, in this case, it may be urged that the black race has become by transmission of immunity partially immune. But these cases which appear to be examples of transmitted immunity may receive another, and a much more simple, explanation. No two children of the same parents are alike in colour of hair, shape of limb, temperament, etc., and they also differ widely in their capacity to receive and combat infection. An epidemic of fever, therefore, will always select to kill those organically most liable to fall a prey to it, while the remnant, having by nature greater power of resistance, not only survive, but may also be calculated upon to produce progeny, on the whole, as resistant as they are themselves. _Man has been selected by the Action of the Microbes of Fever._