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THE PRESENT EVOLUTION OF MAN—PHYSICAL
duced. Here again these special chemical products could not be detected in the blood, and must have been present in such infinitesimally small quantities, that it is difficult to see how they could exert any very marked influence on the activity of the bacteria; whilst, as Flugge points out, our knowledge of the actions of the tissues on foreign bodies of various kinds would lead us to the conclusion that any such material would be very rapidly eliminated. Then Grawitz suggested that in any battle between the cells and the bacilli that may occur in the body during the course of a disease, if the cells can but manage to obtain the upper hand and to destroy the bacteria, they should become hardier, as it were, through the training of the contest, their vital energy and assimilating power should be increased, and they should thus become able to deal in a more summary manner with any organisms with which they might afterwards be brought in contact. Then came Buchner's theory of the inflammatory cause of immunity, which offered another explanation. He argued that bacteria made their way into the body at certain special points, these points, seats of infection, differing in different diseases, and that in consequence of the development of the bacteria, there was a reactionary alteration, inflammatory in its nature, in the tissues, which fitted them for the future to resist the special organism that had previously made the attack; this minute alteration in the function of the special cells at the seat of the invasion enabling them to resist the further action and invasion of the same organism even at a considerably later period. Again, based on the same principles as Grawitz's theory, came the now celebrated Metschnikoff theory. Metschnikoff holds that the protection against the attacks of microorganisms on the body is entirely due to the action of the amoeboid . cells of the body; that these cells are living pieces of protoplasm; that they are constantly taking into their own substance all foreign particles which find their way into the body; that wherever there is an extra demand on their energies, a large number are attracted to the point at which the work is to be
