ACQUIRED IMMUNITY AN ADAPTATION 253 



that use similar food are liable. Some individuals of a species 

 (e.g. man) are immune to disease (e.g. scarlatina), to which other 

 individuals are liable. Many species of animals normally immune 

 to this or that disease, are liable to infection, if the health be 

 lowered as by abnormal heat or cold. Antitoxins ('receptors') 

 may be obtained from the dead or dying cord of a rabbit which 

 was suffering from rabies and would infallibly have perished if 

 it had not been killed ; that is, though receptors are not produced 

 in the living animal in sufficient abundance to neutralize the toxins, 

 and though they cannot, of course, be produced by the dead cells 

 of the drying cord, yet in the latter they are thought to be in 

 abundance. Snake venom, in doses lethal a thousand times over 

 if injected under the skin, confers immunity if swallowed. Whence 

 the receptors in the latter case ? All these facts are inconsistent 

 with the side-chain theory, which with many other bacteriological 

 hypotheses would, I think, never have been formulated had their 

 authors taken the whole of the facts into account had they 

 paused to consider that living nature is a product of evolution, not 

 of miracle. 



423. If the reader bears in mind the facts of adaptation, if he 

 remembers that almost every important character is an adaptation, 

 I think that he must conclude that acquired immunity is not an 

 isolated phenomenon nor a ' fluke,' but essentially a use-acquire- 

 ment entirely comparable to other use-acquirements in the sense 

 that the power of acquiring it has been evolved. I think, also, he 

 will find the strongest reasons for believing that the process by 

 which this habituation is achieved, is, in the case of virulent 

 complex toxins at least, one of reaction from weak to stronger 

 toxins, the weak toxins, the so-called anti-toxins, being nothing 

 other than stronger toxins that have been more or less digested, 

 and in this way chemically altered by enzymes secreted by the cells. 1 



1 All the evidence points to the fact that acquired immunity is especially a 

 reaction to the extra-cellular toxins (exotoxines) those that are secreted by the 

 microbes into the surrounding medium, not retained within themselves. Thus, 

 while an experience of a few days confers immunity to such diseases as smallpox 

 and measles in which symptoms of toxin poison are much in evidence, life-long 

 experience confers no increase of resisting power against tuberculosis or leprosy 

 even when the sufferer is but slightly infected and his natural powers of resistance 

 are but little lowered. Effective antitoxic treatment, therefore, would seem 

 impossible in such cases. Very scanty success, or no success has attended all 

 attempts to deal thus with maladies which, being experienced, do not naturally 

 give rise to acquired immunity in resistant individuals. The celebrated tuberculin 

 treatment of phthisis is a case in point. Tuberculin is prepared from dried tubercle 

 bacilli and contains therefore an intra-cellular toxin (endotoxine). It seems to 



