done; and that these cells, acting on the micro-organisms just as they do on foreign bodies, take them up into their substance, digest and convert them partly to their own uses, and gradually throw into the circulating fluids of the body small quantities of effete substances, which are removed by the ordinary physiological channels. Some observers think, however, that the process is not so simple as it would appear; certain bacteria secrete substances which appear to exert a paralyzing effect on the cells, and may so alter them that they are unable to perform their proper functions; whilst on the other hand, the cells secrete in the performance of their work a material which has an unfavourable influence on the activity of the bacteria. This at first sight is an extremely feasible explanation, but when we come to consider more carefully the conditions under which immunity against diseases is conferred, we find that, although in certain cases an attack of one disease protects against an attack of a more serious and deadly malady, this occurs only within certain definite and well-defined groups of diseases; there appears, therefore, to be something more than a mere general protective influence generated within the body. We must have specific powers of resistance developed in or by the cells in order that they may be able to resist specific bacterial activities, and the effects of specific bacterial poisonous products ."—Bacteria and their Products, p. 369, by Dr. Sims Woodhead.
It is not necessary for the purposes of this work that the question in dispute—i.e. as to how acquired immunity is acquired—should be correctly decided, for, since the power of acquiring immunity is manifestly a most important factor in man's present struggle for existence, our subject only demands that we shall note that it must be undergoing evolution in him. But nevertheless, since the matter is of the highest interest and importance, and since it sheds a clear light on the
