126 THE PRINCIPLES OF HEREDITY 



in greater abundance. But the central mystery as to how 

 this power arises remains unexplained. The hypothesis is 

 founded, unfortunately, on data which cannot be verified, and 

 it is opposed to the facts of evolution to the known parsi- 

 mony of Nature. The primary function of the side-chains is 

 supposed to be nutrition. But in a man there must be as many 

 varieties of these side-chains as there are separate diseases 

 against which he is able to acquire immunity ; for immunity 

 to any one disease does not protect against any other. Even 

 races which are susceptible against this or that disease, but 

 which have had no previous experience of it, must possess 

 the whole budget of side-chains, since all races are capable of 

 acquiring immunity against all diseases in which toxins are 

 abundant. Moreover antitoxin may be obtained from the 

 dead and drying cord of a rabbit which would infallibly have 

 perished of rabies that is, though the side-chains are not 

 produced in sufficient abundance to neutralize the toxins in 

 the living animal, and though they cannot, of course, be pro- 

 duced by the dead cells of the drying cord, yet antitoxin is 

 present in abundance. Clearly the hypothesis breaks down 

 in this instance. It is possible to understand the evolution 

 of a general power of resisting or recovering from infection 

 in the higher animals, surrounded as these are by unicellular 

 beings which would otherwise flourish within them. It is 

 easy to understand in the case of races exposed to particular 

 diseases how this power may be accentuated in particular 

 directions by Natural Selection. But it is not possible to 

 understand how a race which has not had previous expe- 

 rience of measles, for instance, should have evolved side- 

 chains capable of neutralizing the toxins of that disease. 

 It is improbable in the last degree that side-chains evolved 

 primarily for nutritive functions should so exactly hit off all 

 the toxins on earth. 



204 (c). Contrast Ehrlich's hypothesis with the simple ex- 

 planation of immunity set forth in the present work. The 

 latter makes no assumptions except such as can be verified, 

 or such as analogy renders extremely probable. It is in 

 accord with the facts of evolution. It supposes that inborn 

 racial immunity is not due as a rule to any special resisting 

 power on the part of the attacking species, but to lack of 

 adaptation to that particular environment on the part of the 

 attacking micro-organism ; that the inborn immunity occa- 

 sionally displayed by certain individuals of a susceptible race 

 (e. g. man in the case of scarlet fever) is due to a resisting 

 power so high that the disease germs are unable to establish 



