Immunity in Unicellular Organisms 27 



sugar is usually disdained by the yeasts that set up the fermentation 

 of glucose ; it is not difficult, however, to adapt them to galactose 

 which they then attack and transform into alcohol and carbonic acid. 



The Protozoa can be progressively accustomed not only to poisons 

 but also to altered physical conditions. Thus, Dallinger 1 succeeded 

 in raising the temperature of the water in which flagellated Infusoria 

 were growing from 15*5 to 23 C. without causing their death. By 

 prolonging the experiment over several months, he was even able to 

 habituate them to an existence at a temperature of 70 C. In the 

 opinion of Davenport 2 , a view which is shared by many other 

 observers, this resistance to high temperatures was dependent on 

 the abstraction of water from the protoplasm. Dallinger has also 

 observed that in Infusoria that are accustomed to life in hot water, 

 the vacuoles become smaller and smaller and may even actually 

 disappear. 



This adaptation, then, is a property that is very general and wide- 

 spread in the microcosm of the unicellular organisms. It is connected 

 with the intracellular digestion of solid food and with the absorption 

 and transformation of soluble substances. These phenomena, chemical 

 in character, are intimately linked with the irritability of microscopic 

 organisms, which represents one of the fundamental properties of [30] 

 living organisms. 



A Protozoon, which is refractory to a parasite, may protect itself 

 by flight or it may devour and digest the parasite ; another, which 

 acquires a tolerance in regard to a toxin or to a mineral poison, 

 absorbs, fixes and transforms this substance. Consequently, in all 

 these instances of immunity there is a reaction of the living elements 

 of the organism, this being a direct consequence of the irritability of 

 the protoplasm. 



Before an Infusorian retreats from the dead body of an allied 

 organism, before a Protozoon secretes a digestive fluid around the 

 prey it has ingested, before a Bacterium secretes a glairy layer for 

 its defence, etc., these unicellular organisms must receive sensations 

 which provoke the above-mentioned reactions. It is to a celebrated 

 botanist, Pfeffer, that we owe the most important researches on this 

 irritability of unicellular organisms, researches which may be summed 

 up in the general statement that this property is subject to the psycho- 

 physical law of Weber-Fechner. Pfefler, by the observation of the 



1 Journ. R. Micr. Soc., London, 1880, m, p. 1. 



2 Davenport and Castle, Arch.f. Entwickelungsmech., Leipzig, 1895, Bd. n, S. 227. 



