THE PRESENT EVOLUTIOX OF MAN — PHYSICAL 209 



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 eftete substances, 

 which are removed by the ordinary physiological chan- 

 nels. 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 pro- 

 tects against an attack of a more serious and deadly 

 malady, this occurs only within certain definite and 

 Avell-defined groups of diseases ; there appears, there- 

 fore, to be something more than a mere general pro- 

 tective 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 Fro- 

 ihtcts, 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 



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