THE CONCEPTUAL WORLD 37 



toxins first administered have disappeared. We must 

 imagine, therefore, that the anti-substances produced 

 originally by the reaction of the toxin are produced 

 again and again by the tissues of the susceptible 

 animal, for the latter may resist repeated infections, 

 that is, repeated doses of toxin, without illness. But 

 then the tissues of the animal body are transitory 

 substances and they do not persist unchanged. 

 Muscles, glands, connective tissues, even nerve-fibres 

 and nerve-cells undergo metabolism, and the chemical 

 substances of which they are composed break down 

 into the excretory products, pass out into the blood 

 stream, and are eliminated from the body ; while at 

 the same time these tissues are continually being 

 renewed from the nutritive substances in the blood 

 and lymph. It is the organisation of the tissues 

 their form and modes of reaction that endure, but 

 the material substances of which they are composed 

 are in a state of continual flux. Yet the organisation 

 of these tissues does not persist unchanged, for it is 

 continually responding to new conditions experienced 

 by it. The reactions that occur when a toxin is 

 administered to a susceptible animal affect the organis- 

 ation of its tissues in such a way that the latter acquire 

 the capability of producing antitoxins which may if 

 we like to say so neutralise the toxins that enter 

 into them when they become infected. The reaction 

 endures. But this is a different thing from saying that 

 the process is a physico-chemical one alone. 



This is what we must understand by the duration 

 of the organism. Everything that it experiences for 

 the first time persists in its organisation. It acquires 

 the ability of responding to some stimulus by a definite, 

 purposeful reaction, the effect of which is to aid it in 

 its struggle for existence ; and this reaction, once 



